[[quoteright:350:[[Manga/FullmetalAlchemist http://static.tvtropes.org/pmwiki/pub/images/fma_equivalent_exchange_8912.jpg]]]]
[[caption-width-right:350:[-You can't make a metal weapon without a little metal floor.-] ]]

->''"Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost. That is alchemy's first law of '''[[TropeNamer Equivalent Exchange]]'''. In those days, we really believed that to be the world's one, and only truth." ''
-->-- '''Alphonse Elric''', [[ArcWords numerous times throughout the series]], ''Anime/FullmetalAlchemist''

Power has [[MagicAIsMagicA consistency]].

Power serves a [[FunctionalMagic function]].

Power '''always''' comes [[PowerAtAPrice with a price.]]

To acquire the ability to perform something, induce motion, bring change -- to bring something into existence, [[MakeAWish grant a wish]], [[EmpathicHealer heal a loved one]], or even [[BackFromTheDead bring someone back to life]] -- someone must [[PowerSource give up another thing of equal value]]. This trope has a simple dramatic purpose: it prevents the pitfalls of a typical {{Story Breaker Power}} by adding laws and the elements of choice and sacrifice.

Would you give up the gifts, {{love|Interest}}s, and [[CastFromHitPoints life]] you possess to get what you desire? Beware the allure of [[DealWithTheDevil Desire]], however, because [[ComicBook/TheSandman it doesn't always work in your favor]] -- [[BeCarefulWhatYouWishFor people may not truly understand]] [[LiteralGenie the full effects of what they desire]], and the gift itself can end up [[BlessedWithSuck becoming its own cost.]] King Midas thought of the Golden Touch as a great boon, but it ultimately robbed him of his greatest treasure: [[ThickerThanWater his daughter]].

This trope has [[TruthInTelevision a layer of truth to it]], given the [[http://en.wikipedia.org/wiki/First_law_of_thermodynamics First Law of Thermodynamics]]. Building in the cost to the boon usually results in a FantasticFragility.

The principle of Equivalent Exchange typically says the object or goal a person will trade for must have equal value to what the person trades with. Who and what determines this "equivalency" [[HouseRules varies from story to story]]:
** Does the Exchange measure emotional value, monetary value, or both? (Someone might find a penny handed down from father to son as worthless as if he found it on the ground, even though the father and son would consider it emotionally valuable.)
** If a wizard [[RitualMagic ritually]] sacrifices a cat to get some magical mojo, what determines how much power the wizard receives: the value of the cat's life to the wizard, the value of the cat's life to the cat, or the value of the cat's life to a deity?
** In Real Life, the First Law of Thermodynamics follows this dilemma, since it only measures mass and quantitative scale. An action only produces an equal and opposite reaction; a sacrificed cat will only release the energy of a cat, burning a human will work nearly the same as burning a human-sized carbon statue, and deconstructing an atomic nucleus to perform alchemical transmutation requires a nuclear reactor. In fiction, we can assume that [[OurSoulsAreDifferent souls]] become necessary to harness cosmic-scale energy or contain magical properties that don't follow the laws of thermodynamics (hence the necessity for HumanSacrifice as a viable form of {{unobtainium}}).

Compare this trope to BalancingDeathsBooks and MutualDisadvantage. The tropes known as CallItKarma and TheGoldenRule attempt to apply this law of physics into ethics and morality.

----

!! Examples of this trope include:

[[foldercontrol]]

[[folder:Anime and Manga]]
* ''Manga/FullmetalAlchemist'':
** This franchise serves as the TropeNamer for anime and manga, as it makes a big deal out of the principle in its plot. In regards to alchemical transmutation, the law basically works like the Newtonian law of conservation of matter: matter and energy cannot be created or destroyed in a reaction. (It also helps that IsaacNewton, one of the {{Trope Codifier}}s in Real Life physics, considered himself an alchemist and formulated the basic principles of this trope.) The energy used to perform Amestrian alchemy comes from either tectonic shifts (the manga and ''Brotherhood'' anime) or [[spoiler:the souls of our world's deceased]] ([[Anime/FullmetalAlchemist the 2003 anime version]]). Xingese [[InsistentTerminology alkahestry]] relies on reading and directing the Earth's natural energy through its mantle.
** Ed gets a wonderful chance to explain the principle at the start of the story. While talking with a religious zealot who believes her leader can reincarnate someone, Ed lists all the raw materials that exist in the human body -- noting that anyone could buy a "body" with pocket change -- before he explains that even with the materials gathered together, someone still can't make a human life because the exchange still doesn't have equivalency. He comes to a simple conclusion: you need a soul to complete the transmutation, and the right one at that. [[spoiler:If you don't care about what soul you want, you can make a massive army of homunculi...]]
** The legend of the Philosopher's Stone says it enables an alchemist to perform any type of alchemy without cost (including human transmutation). [[spoiler:The Elrics learn that even the Stone has its own form of Equivalent Exchange: to make one, someone has to sacrifice a hell of a lot of people first. A massive human transmutation collects the souls of those sacrificed to make it and turns the stone into a massive portable power supply -- one which lets its user create matter from the massive energy stored inside it, even if the energy seems to come from nothing.]]
** One of the central tenets of the series -- and a sign of the development of Alphonse and Edward as people -- shows that [[spoiler:no matter how much they try and apply the rule of Equivalent Exchange to their lives, they always find something missing]]. At the end of the manga and ''Brotherhood'', they vow to [[spoiler:give eleven back to every person or thing they take ten from, thus making the world richer.]]
** Alphonse makes a crucial logical conclusion about the principle near the very end of the series. If A equals the cost of B, then the reverse should hold true: [[NowDoItAgainBackwards you can exchange B back for A]]. [[spoiler:Once he figures it out, Al gives his soul ''back'' to Truth so Ed can have his right arm again and defeat Father once and for all. Ed does not take this development well.]]
** [[spoiler:Hoenheim likewise believes the theory has a flaw only in the ''opposite'' direction: "Even if I lived forever, I'd never give enough to have earned my sons/family."]]
** The 2003 anime displays the point about how Equivalent Exchange [[DeconstructedTrope never quite works out in life in a brutal fashion]]: whenever Ed and Al try make the world a better place, their work ends up making things worse -- and all because of their belief in Equivalent Exchange. (Numerous variables in life always affect the outcome, just like Chaos theory.) [[spoiler:Dante's BreakingSpeech to Ed towards the end, where she picks apart the theory and exposes its flaws, drives this point home]].
* ''VisualNovel/FateStayNight'' (and its [[{{Nasuverse}} multiverse]]) contains this -- energy must be taken from somewhere before it runs through the [[FunctionalMagic Magic Circuit]], though in most cases, Prana is either generated by the magus ([[LifeEnergy Od]]) or taken from the environment ({{Mana}}). Rin can launch A-rank attacks in seconds because she taps directly into her jewels (where she has been saving [[{{Mana}} Prana]] for ten years) instead of gathering it slowly for a minute or three.
** This is a plot point for all three scenarios in the game, as protagonist Emiya Shiro has to fight, but the only magecraft he is skilled at is far beyond his capabilities. Whenever he uses it excessively, blood loss (and death) is inevitable.
** One exception is Rin’s Jewel Sword Zelretch, which lets her use strong attacks indefinitely by absorbing mana from alternate realities. The sword is explicitly made with "True Magic", which breaks the usual rules of Equivalent Exchange, and which only about five people in the world are capable of using.
* In the multiverse of ''TsubasaReservoirChronicle'' and ''XxxHolic'', the price for wishes granted by Yuuko works this way. Some characters are much more wary of the possible implications of this than others. In particular, Kurogane and Doumeki will only ask for Yuuko's help if it is flat-out impossible for them to solve the problem under their own power. This is prudent, as false-hearted or impulsive wishes usually result in nasty karmic payback. Yuuko also has rules about not dealing in wishes of value equal to a human life, as that would implicate the wish-granter in murder.
** For the record, Yuuko accepts items or intangibilities that have both real and emotional power, which include both living and non-living: Kurogane's sword is both a precious reminder of his family and a powerful weapon; Fai's tattoo is imbued with powerful magic for his own safety; and Syaoran and Sakura pay by giving up a portion of Sakura's memories - those which contain Syaoran. This means their previous relationship is sacrificed forever. Syaoran has been edited out of the memories the feathers contain. For example, Sakura can remember the events of her last birthday party, but one seat is mysteriously empty, and there's an odd pause in the dialogue whenever someone would have otherwise said Syaoran's name. One time Sakura almost manages to put two and two together, but passes out and loses all memory of her deduction due to this price. It's not flat-out impossible for Sakura and Syaoran to have a relationship after paying the price, but it has to be rebuilt from the ground up.
** Interestingly, you apparently don't need someone to trade with, just need to have a payment to be given in exchange for a desired end. [[spoiler: Clow and Yuko gained entry into the cycle of reincarnation for the clones by giving their lives up to... nothing in particular. Watanuki and Syaoran Jr. escape from a pocket dimension in exchange for the inability to leave The Shop and the inability to remain in one world for very long, respectively, again not actually selling it to anyone in particular]].
** It eventually is revealed that whoever is the owner of the Wish Shop is essentially contractually bound to live by the EquivalentExchange principle at all times, lest they risk getting magically induced injuries from the shop itself as compensation. It's sobering when you realize that this is the primary reason for why Yuuko has made payment requests for the slightest things, even to friends, because no matter how much power she has, she ''can't'' give anyone anything without needing something from them in return... Except for feelings, which are explained to be the only things people can give without a price.
* ''WitchHunterRobin'' had a doctor whose special witch power allowed him to transfer LifeEnergy from one person to another. He used this to heal patients -- and took the necessary energy for his power from mob bosses. [[spoiler:He ultimately decides to save his human partner by sacrificing his own life.]] Considering that the witch hunters kill most witches they go up against, with those captured alive [[spoiler:taken for experimentation]], he definitely made the right decision. Since the STN-J hunts all Witches regardless of whether they can morally justify the use of their powers or not, Robin ends up learning about the holes in her philosophy by going on a less defensible (and fairly nasty) witch hunt.
* ''HunterXHunter'' has Nen abilities. It's a special ability invented by a user by using Nen, and it can range from simple super strength to shrinking cloths to memory reading to [[ImprobableWeaponUser vacuum cleaners and books and bank accounts]]. Each of these abilities have unique powers that the user make up in his mind during conception. The more powerful you are, the more powerful ability you can create. The thing is, the more powerful the ability is, the more restrictions the user must needs impose on it as well. Examples are:
** Kuroro's Skill Hunter ability allows him to steal the Nen abilities of others and record them into his book, thus robbing them of that ability forever. It's an awesome move, but there are many rules. First, he must see the ability in action personally. Second, he has to ask the target about the ability and be answered. Third, the victim's hand must touch the palm print of the cover of the book. Lastly, all of the above must be done within only one hour. Otherwise, it's back to step one. Now, if you get the ability into the book, it's all well and good. But another rule is that if the original user dies, the ability vanishes from the book forever, and he can't use it anymore.
** Kurapika's Chain Jail is a long chain used to wrap around the opponent's body. Not only that, it shuts down their aura, and prevents them from using Nen abilities. However, because it is an overpowered ability, he had to impose the condition that he could only use it on a member of the Genei Ryodan. If he uses it on a person who is not a member of the group, he dies. This causes problems because there are only at most thirteen members of the Genei Ryodan at any given time.
** Alluka Zaoldyk's wish-granting powers are an especially sadistic take on this since the price of the wish is passed on to someone else instead of the person making the wish. If that person can't fulfill Alluka's requests (and the requests can be as severe as ''ripping out your own brain'' if the preceding wish was significant enough), then that person and that person's loved one ''die''. Depending on the severity of the wish, everyone that person has ever ''met'' is also at risk of dying instantly. [[spoiler:It is later revealed that this only applies to wholly selfish wishes. A wish to heal someone for example can be granted for free.]]
** Kaito deliberately made his Nen ability annoying and inconvenient (the slot machine clown won't shut up, the weapon it summons is random, the scythe form can only be used for one technique, and the weapon can't be dismissed until it's used, etc.) to increase its power. He still complains about it.
** Franklin of the Genei Ryodan can fire very powerful Nen bullets from his fingertips. He made them that powerful by ''mutilating his own fingers''.
** Shizuku, also a member of the Genei Ryodan, summons a vacuum cleaner that can suck up anything as long as it's not something Shizuku considers to be alive, and only the last object sucked up can be recovered.
** In chapter 305-307, [[spoiler:Gon gives in to his rage and despair]] and "uses everything" to defeat an incredibly powerful enemy. He basically sacrifices his future to rapidly age himself into an adult. This gave him the power to crush his foe, but it left him on the brink of death. The Nen curse he placed on himself to obtain that brief surge of power was too strong for the Hunter Organization's Nen Exorcist to even ''consider'' removing. [[spoiler:It took the aforementioned Alluka's power to remove it.]]
* ''HibikiNoMahou''. If you want to use magic properly, you need to sacrifice an aspect of your being, like ability to dream, age, or memory; no wonder why magic practitioners are declining.
* Although it's not too much of a regular magic as it's more of a one-use, people at ''Anime/HellGirl'' can send one person they dislike immediately to Hell by contracting Enma Ai. The price is their own soul going to Hell when they die. That's the reason why many people hesitate in first place: going to Hell is a really high price. Out of spite, they send their victims into Hell anyways. The lesson is to learn to control your emotions and hatred? or YouCantFightFate? [[LostAesop Who knows?]]
* It is the premise of ''Manga/TheLawOfUeki''.
* ''{{CMB}}'' has the rather interesting case in which [[CloudCuckooLander Shinya]] requires a price for solving a mystery; luckily he's very easily bought off so long as it is interesting.
* ''CodeBreaker'': It turns out that [[spoiler: Ogami's powers come from a DealWithTheDevil: for every upgrade he gets, he loses one of his senses. He's already lost his sense of taste, but "fortunately" Code: Emperor has chosen to take his newly acquired sound powers rather than his hearing... although there's still five upgrades to go...]]
* The second arc of ''{{Naruto}}'' started off with Gaara dying, but he was an immensely popular and critical character so he needed to be revived. Enter Chiyo, who has a ninjutsu capable of reviving the dead. To avoid this being used to prevent every death in-series, the jutsu was equivalent exchange: the user sacrifices their own life to resurrect another.
** This is also how [[spoiler: Nagato/Pein dies, a man with the bloodline trait giving him magic eyes equal to the strongest ninja ever to exist, it goes to show how powerful he was that he killed ''thousands'' and was able to revive them all to the last one before succumbing to the jutsu.]]
* ''Manga/AhMyGoddess'' has the Law of Conservation of Happiness.
** Eventually, granting too many wishes within a short period of time means that someone ends up suffering some form of bad luck.
** This is also how wishes granted by demons run. They usually come with a myriad of attached strings that cause grief and misfortune to the person making the wish.
* ''Anime/PuellaMagiMadokaMagica''
** It is an in-universe theory that the wishes that get fulfilled in exchange for the girls becoming magical girls are guaranteed to cause an amount of despair equal to the hope they generate. A SelflessWish for the sake of others would eventually lead to the wisher suffering in turn, unable to enjoy the very wish she herself generated, especially when the Magical Girl persona is forcing the wisher into isolation. "Hope and despair balance out to zero." Although this is never treated as anything but a hypothesis, and more likely to be based on one of the themes of Goethe's ''{{Faust}}'': As part of the DealWithTheDevil, should the Faust ever reach the highest state of happiness possible for humanity, he will immediately die and be consigned to hell.
** Defying this trope [[spoiler:is the true goal of the Incubators. Magic generated through emotion breaks Newtonian Equivalent Exchange, and the Incubators are using the additional energy to offset the death of the universe. However, for the Magical Girls themselves, [[PoweredByAForsakenChild the Equivalent Exchange of hope and despair still remains to screw them over]].]]
* In ''LightNovel/ACertainMagicalIndex'', once one becomes an [[PsychicPowers esper]], they can never use magic, and vice-versa, without suffering extreme damage to their bodies.
** To gain the incredible powers of God's Right Seat, a magician must give up his or her ability to use normal spells. [[spoiler: Acqua of the Back gets around this because his Divine Mother's Mercy removes limitations and secondary conditions. Fiamma of the Right gets around this by brainwashing Index to do the spells for him.]]
* Explicitly averted in ''TowerOfGod''. [[AppliedPhlebotinum Shinsoo]] is potentially limitless in power and potency, and can be created and destroyed. Killing, shrinking, even creating life are made possible. There's only one catch. To even gain control over one control unit of Shinsoo, one needs to rigorously train for ten years. Every further technique used takes dozens of years to master. Some people aren't even capable of doing so, since Shinsoo would instantly mangle their body. Why does it take so long? Because a single fuck-up with a greater amount of shinsoo is enough to completely obliterate you. It's dangerous stuff, kids. Handle with caution.
* While most of the magic on ''Anime/OjamajoDoremi'' doesn't have this in place (at least for full-fledged witches and wizards), the forbidden types of magic ([[EmpathicHealer Healing]], MindControl, [[BackFromTheDead Raising the Dead]]) all have this as part of why they are forbidden.
* This trope gets applied vaguely in ''Manga/DeathNote'': though no tangible price or punishment exists for a human who uses the [[ArtifactOfDeath Death Note]] (according to Ryuk), those who use it always end up miserable. [[AGodAmI Light]] vows at the beginning of the story to break this pattern; by the end, [[spoiler:he loses everybody who ever cared about him in any way, then dies in a bloody heap as he begs [[HoistByHisOwnPetard Ryuk to spare his life]].]] Everyone else who writes in the Death Note [[spoiler:also ends up either miserable or dead]].
* In ''Manga/{{Bleach}}'', a Hollow can give up his or her HealingFactor in exchange for extra strength.
[[/folder]]

[[folder:Comic Books]]
* In the British ''ComicBook/TheTransformers'' comic, if someone travels back in time, someone from the destination period vanishes into Limbo while the traveler is there. Attempts to avoid this will eventually get you eaten by a time warp.
* In the Pre-Crisis DCU, the Guardians of the Universe purged themselves of evil, only to find that the evil had to go ''somewhere''. They [[SealedEvilInACan sealed it in the same universe where they sent most of the magic]] to ensure that the [[TheGoldenAgeOfComicBooks Golden Age]] GreenLantern would be around to deal with it.
* {{Storm}}'s weather-controlling powers in the ''Comicbook/{{X-Men}}'' comics are given a similar limit to explain why she can't turn the world into a paradise, or at least bring relief to disaster-stricken areas. The first time she tried that, she successfully ended the drought in her village - only to discover that, since the moisture she'd drawn upon had to come from somewhere, she'd caused even worse droughts to strike the rest of the world.
* In PS238 there's Toby Marlocke, who has RealityWarper powers with a catch. Every time he tries to do something more complicated than self-levitation, there is an equivalent unintended result. This can range from funny (absorbing the kinetic energy of a super-powered soccer kick caused tulips to sprout at the goal line) to rather disturbing (creating a superhero persona for himself caused the creation of a new super''villain''). Part of the reason for this is that he got his powers in the first place as part of a deal between the cosmic forces of Order and Chaos, and both sides insisted that the balance should be preserved...[[/folder]]

[[folder:Fan Works]]
* In ''FanFic/HarryPotterAndTheMethodsOfRationality'', Professor Quirrel claims all magic works like this. "Normal" magic works by sacrificing the constantly-regenerating [[{{Mana}} pool of magical energy]] all wizards and witches have. To contrast, the price of any Dark magic is permanent, and dearer than the price of "normal" magic.[[/folder]]

[[folder:Film]]
* In ''Film/ConanTheBarbarian1982'', Conan is mortally wounded by his crucifixion on the Tree of Woe, so his lover Valeria convinces the Old Wizard to work a healing spell to save him from the brink of death. The Wizard warns that the gods demand a price for this sort of thing, and she says she'll pay it. When she's later shot with a snake arrow by Thulsa Doom, she decides that her death is her payment. Whether that's true is not revealed, but it seems logical given the [[CrapsackWorld bleak universe]] of the film.
* The fountain in ''Film/PiratesOfTheCaribbeanOnStrangerTides'' takes life from one to extend another's. However, the rate of exchange isn't exactly equal. The one on the receiving end gets the person's entire natural lifespan, regardless of their age or actual life expectancy, meaning that the recipient is going to get around 100 years from the deal no matter the sacrifice.
* The magic [[TimeTravel time traveling]] scepter of ''Film/TeenageMutantNinjaTurtlesIII'' required an equal number of people (of roughly the same weight, which didn’t come up) to travel in each direction. While the movie is [[FanonDisContinuity not all that popular]], it did make for [[HilarityEnsues some interesting events]] going on while the Turtles were back in Feudal Japan.
* In ''Film/TheCovenant'', every time a gifted teenager uses his/her RealityWarper powers (which are highly addictive), the person gives up some [[spoiler:youth]], as we see later in the movie.
* Rumplestiltskin's "Ogre for a Day" contract in ''WesternAnimation/{{Shrek}} Forever After'' works by taking a day from the signer's past (in this case, Shrek). The day Rumple chose was [[spoiler:the day of Shrek's birth, so once the day is over, so is he]].
* In the first part of ''MerlinsShopOfMysticalWonders'', the main character (a jerkass StrawCritic) gains magical powers at the expense of his life force.
* A relatively benign example of this trope occurs in ''Film/TheSecretOfTheMagicGourd''. Rather than a life for a life, the Magic Gourd uses magic to swap Wang Bao's blank failure of a test paper with the high-scoring test of a fellow classmate. Being his typically dumb self, Bao Hulu doesn't even bother to change the name at the top of the test, which gets Wang Bao in trouble for cheating.
* In ''[[Film/MenInBlack Men In Black III]]'', J goes back in time to prevent [[spoiler: K's death]]. He is told that the event he is trying to avoid is destined to happen, and that "Where there is death, there must always be death.". [[spoiler: Due to J's meddling, K is saved, with J's dad TakingTheBullet.]]
* Since the precise details of time travel in the ''Franchise/{{Terminator}}'' series are left vague and we never see the aftermath of departure, one possibility is that the Time Sphere doesn't [[SphereOfDestruction destroy]] everything around the traveller, but merely transposes the future and past in their location.

[[/folder]]

[[folder:Literature]]
* Christopher Paolini's ''Literature/InheritanceCycle'': The ''[[FunctionalMagic entire magic system]]'' of the series is wrapped up in this. Doing something with magic takes the same amount of energy as doing it without magic would have done. And if you try casting a spell that takes up more energy than you have, you die.
** This is what he says, but at one point, Eragon breaks the calves of dozens (maybe hundreds) of soldiers and doesn't die. And how much energy does it take to make a blue fireball without magic?
*** The color of the fireball doesn't indicated how hot it is, it just means his dragon is blue. Brom says it out right in the first book, for dragon riders their magic will be the color of their dragon. So it would take no more energy then a fireball that would "naturally" burn red.
** And certain spells are just flat-out impossible, presumably because they take far too much energy, or simply cannot be done. For example: Eragon is warned to never try to bring people back from the dead; beyond death, there's just something that magic has no effect on; an attempt would drain the mage of all his life in one go and accomplish nothing. Trying to see the future or the past is a bad idea as well.
*** It's actually explained in the books that if you're trying to see the future, it's a lot like scrying, in that you need to know exactly what's going to happen, down to the last detail. But if you already know what's going to happen, then scrying the future is functionally pointless.
** Although in the second book, the young woman running the Varden finds some wiggle room in the rules: doing something with magic takes less ''time'' than it would otherwise, therefore magic-users can outperform in tasks which are complicated but low-energy. And that's the story of how the Varden climbs out of a financial hole by [[spoiler: producing and selling finely made lacework for ladies garments]].
* This also comes up a lot with magic in the ''Literature/{{Discworld}}'' books, where it is referred to as the Law of Conservation of Reality. For example, to teleport someone from one side of the disc to another, you may need to have an equivalent weight to teleport back to where they came from. This is mainly to deal with conservation of momentum; because the Disc rotates, different points on its surface move at different velocities relative to the Hub. Teleporting without such a counterweight means that if you move very far, your velocity relative to your immediate surroundings tends to kill you. But you can do it!
** This is still an imperfect science, and when the wizards of Unseen University try it to retrieve Rincewind in ''Interesting Times'', he's hit on the head by all the crap they piled up to equal his weight, going the other way.
** It comes up more often with conjuration than with transportation. If you wanted to make, for example, a loaf of bread appear, the casting thereof would have to expend all the energy that went into making the bread--so, growing the grain, grinding it into flour, mixing the dough, all the heat it took to bake it--or else you'd have a loaf of bread for about half a second and then it would vanish again. It is also described that the magical act of creating that loaf of bread as a permanent thing takes so much effort that it might cause the wizard's brain to forcibly evacuate his skull through his ears. So mostly they just don't bother.
*** Another anecdote describes how using magic to levitate a book from a high shelf to lay it down in front of you will fatigue you as much as actually climbing a ladder to take the book directly.
** Esk mentions it outright.
-->''Esk: I don't think magic works like that. You can't just make things happen, there's sort of- like a seesaw thing, if you push one end up, the other goes down.''
-->''Gulta: I can't see Granny on a seesaw.''
** In a non-magical example from ''Discworld/{{Snuff}}'', the only item Vimes can offer Tears of the Mushroom for the loan of her precious unggue pot is the equally-precious iconograph of his son which he keeps in his wallet.
* In the non-{{Dragonlance}} book ''Literature/TheDeathGateCycle'' by MargaretWeis and Tracy Hickman, the same problem came up. When you brought someone back to life, be it a full resurrection or just as a zombie, somebody else, somewhere, died. One of the two competing races nearly wiped themselves out this way.
* ''{{Vurt}}''. Things are swapped between the real world and the vurt world on the basis of their value. The characters are keeping a weird tentacled creature because it was somehow switched with the protagonist's sister, and they're trying to figure out how to get her back. At one point, he wants to bring a object back to reality, so he leaves something of sentimental value behind.
* Blood-magic in George Martin's ''Literature/ASongOfIceAndFire'' works this way;
** Near the end of the first book, [[spoiler: Danaerys sacrifices her husband's prize stallion and her unborn child to save her husband's life]] -- and gets badly screwed by the exchange, since the mage had played upon her desperation [[spoiler: to save Drogo's life, and convinced her that the only life required to restore him was the horse]]'s. Later, she uses the same principle to [[spoiler: hatch three dragons from their fossilized eggs]].
** Melisandre kills [[spoiler:Renly Baratheon with a nigh-unstoppable intangible assassin, [[FanNickname dubbed a "shadowbaby" by fans]]]]. Although the details are never quite revealed, it seems to involve [[DeusSexMachina getting herself pregnant]] and sacrificing the life of the ([[spoiler:royal-blooded]]) unborn child. There are also hints that the spell was at least partially CastFromHitPoints [[spoiler:on Stannis' part]].
** After Arya has saved the lives of Jaqen H'ghar and two other guys, Jaqen explains to her that he is now owing her three assassinations - one for each life Arya has by her actions denied the Red God to claim.
* The magic system in Eric Nylund's ''Pawn's Dream'' works through a variation of Equivalent Exchange, where opposite elementals must be present, but it varies whether users need to trade them or simply summon or banish both. Either way, most of the skill in magic is based on letting both elements flow freely.
* ''Literature/TheSagaOfRecluce'' series by L. E. Modesitt Jr. is set in a world of Chaos and Order Magic, both of which must be carefully balanced--at times, overuse of either, or just too much Order or Chaos concentrated in one area, has shifted the entire planet's weather patterns, caused volcanic eruptions, and other disasters.
** Despite that the balance is well-known in-universe, it didn't stop people from trying to cheat. Recluce itself, for example, was protected by a navy of Order-infused ships...and every time they replaced one, it was with a larger, more powerful ship that required more Order.
* The novels in the Franchise/StarWarsExpandedUniverse emphasize that the [[TheDarkSide Sith]] path is one of sacrifice, i.e., ''demolishing'' attachments instead of the Jedi path of ''avoiding'' attachments. Since the Sith draw power from pain and rage, easy or pleasant trade-offs aren't in the picture.
** Which makes the Emperor's plan of either 'Luke kills his father' or 'Anakin kills his son' at the end of ''Film/ReturnOfTheJedi'' make a lot more sense.
* Creator/OrsonScottCard is very fond of this trope and said that it is a practical necessity for a fantasy story driven by magic, writing in ''How to Write Science Fiction and Fantasy'' that without it, the "characters become gods...and there is no story". In that book, he proposes some grotesque systems of magical exchange, including killing (a human grants you more power than an animal; a child grants more power than an adult) or losing body parts for power (your own body parts fall off; or someone else's fall off, but they must be given willingly; or body parts of the one you truly love the most fall off; or a random person's fall off, but they tend to have a connection to the spellcaster). And as he mentioned in ''How to Write Science Fiction and Fantasy'', Card actually wrote a novel in such a setting: ''Literature/HartsHope''.
* In the ''Literature/{{Deryni}}'' series, magic is physically taxing, with more powerful workings (say, Calling to another Deryni over a great distance, particularly when the other Deryni doesn't expect it and isn’t helping to bridge the gap from the other end) causing more fatigue. Fatigue-banishing spells exist, but they cannot be re-used indefinitely, and the rejuvenation they provide can be quickly lost if the person experiences more stress. Training also seems to extend endurance within limits, much as people who physically train increase athletic endurance to a point. Tiercel makes this analogy: “...you're flexing abilities you've never used before. You have to build up your endurance. I'll bet you've got a headache just from this afternoon's work.”
* In the ''Literature/ColdFireTrilogy'', magical energy is released by sacrifice. The amount of energy gained is directly proportional to how much the sacrificed object was worth to the sorcerer. This is one of the facts used to show the [[MagnificentBastard Magnificent Bastardness]] of Gerald Tarrent, who sacrificed his entire family to gain immortality. The spell wouldn't have worked if he hadn't loved his wife and children very, very much - and he killed them ''anyway''.
** It's later explained that the sacrifice which made him immortal wasn't actually his wife and children. It was ''his own humanity'', which he lost through the expedient of killing his wife and children.
*** And if he ever tried to act like a compassionate human being again by engaging in an act of life or Healing, his immortality would be forfeit.
** Another huge example in the backstory explains why humanity is stuck on Erna without any of the technology that got them there in the first place. [[spoiler:One of the expedition leaders, Ian Casca, sacrificed the colony ship and most of its advanced technology to make the ''fae'' into something humans could harness and control. In exchange for immediate survival, Casca gave up any chance for the colonists to escape the damn planet. The colonists killed him when they discovered this.]]
* In the early ''AnitaBlake'' stories, one must kill a living thing to create undead. Anita routinely uses goats for that purpose. Of course, most magic is swallowed up by DeusSexMachina as the series moves forward. (She has kept the goats out of that part.)
* This is a rule of magic in the ''Literature/TortallUniverse'' by Creator/TamoraPierce, demonstrated in several ways.
** In ''[[Literature/TheImmortals Wolf Speaker]]'', a man is turned into a tree and we are told that, somewhere in the world, a tree is now a man. (Pierce later wrote about him in a Creator/BruceCoville anthology).
** Words of Power differ from regular use of TheGift in needing equivalent exchange. You can do more minor stuff rather easily. However, for example, destroying a powerful magical barrier with Words would have caused a major natural disaster elsewhere.
** Additionally, we find out in ''Literature/ProtectorOfTheSmall'' that the use of the Dominion Jewel to prevent an earthquake (at the end of ''Song of the Lioness'') cost the fertility of all the grain in the kingdom. They had to import all their bread and seed wheat the next year, and the kingdom was in debt for years paying it off.
* In the ''YoungWizards'' series, "spells" are essentially equations written in a magical language and equations always have to balance, one way or another. In one book, a character holds off the BigBad with a [[BeehiveBarrier shielding spell]] which is later explained to have been fueled by a year of the character's life per strike. (The Devil hits ''hard''.) In another, that same BigBad is [[SealedEvilInACan sealed away]] by a ritual that requires a willing sacrifice; one character attempts to take the place of the intended victim [[spoiler:and a third actually does]]. It's implied that this event might have come to pass because of events in the prior book: to cast a spell they couldn't have powered alone to seal away an ArtifactOfDoom they used a "blank check" spell in which they essentially promise that the power they use will be repaid at some unknown date in the future.
* The "death magic" in LoisMcMasterBujold's Literature/{{Chalion}} series is like this: it's a way to murder someone at a distance, but as part of the spell's workings, the person casting the spell will die, too. In addition to this little drawback, it's [[spoiler:believed to be]] very difficult, and requires animal sacrifice as well. Plus, the God of Executioners has to be invoked and [[spoiler:he only lets the magic work when the victim ''deserves'' to die.]]
* The ''BarryTrotter'' parody of the ''Literature/HarryPotter'' series mentions this in the sequel, where everyone has graduated and started families [[spoiler: (despite being effectively proven wrong by the final book)]]. It's only then that Barry learns that to summon something with magic is actually just teleportation, thus "created" items always come from somewhere else. This coincidentally results in a poor sap whose possessions always go missing on Barry's birthday.
* ''Literature/BrokenSky'':
** This series has "spirit stones", which grant people special abilities but drain their energy when used, placed into the spines of characters. Ryushi, for example, can emit a powerful blast of energy, but it drains him to the point of collapsing to do so.
** We're later introduced to people with healing spirit stones. They can heal the wounds of others by taking them on themselves, although the healers have a slightly improved healing rate.
* In ''Literature/TheNeverendingStory'', AURYN grants humans the ability to make their wishes come true, by rewriting reality in Fantastia so that it always was so. In exchange, AURYN takes away a memory of the Outer World from the human with each wish. They start off reasonable enough (trading the memory of being fat and scared for a more heroic figure), but eventually descends into taking more and more precious memories, no matter how selfish the wish is (the memory of being from the outer world, the memory of one's parents, the memory of one's own ''name''). In the book, the wishes don't even have to be spoken, either. If you want something badly enough, AURYN will sense it, grant that wish and take a memory with no effort on your part.
* A system of Equivalent Exchange is enforced in the ''Literature/NightWatch'' books. Basically, the forces of good and evil have a treaty regulating and limiting their actions. When one side uses their power to interfere with humanity, the treaty demands that the other side receive an equal intervention. Hence a Light mage can heal someone, but that gives a Dark witch the right to curse someone. The system works overall, with most "Others" (the series term for supernatural beings) willing to go through the proper channels to get licensing for using their powers. (For example, vampires annually receive a license to feed on a living human, though not all use them.) If an Other breaks the rules, the Watches (the police of the Others) will locate and punish them (with most crimes being sentenced to death). If a member of the Watches breaks the rules to a relatively small extent, they can offer the other Watch an equal intervention as a compromise.
* In Creator/RobertSilverberg's early novel ''The Time Hoppers'', time travel is done by exchanging matter between the present and the past; when a human is sent back, an equivalent mass of air has to be brought forward.
* In Cornelia Funke's ''Literature/TheInkWorldTrilogy'', characters can be read out of books into the real world, but not without someone or something from the real world taking their place
* The three magic systems from ''Literature/{{Mistborn}}'' work this way.
** Allomancy requires the wielder to consume fragments of metal and "burn" them in their stomachs to power certain effects; some of these metals are considerably rarer than others, and the metal duralumin acts as a catalyst for burning whatever other metals are in the user's stomach ''[[LimitBreak very, very fast]]''.
** Feruchemy allows the wielder to temporarily drain him or herself of a certain quality or ability - memory, skill, age, health, senses - store it in a piece of metal, and retrieve it later as needed (for example, a Feruchemist could temporarily become very weak, and then later "tap" the stored energy to acquire superhuman strength).
** [[BloodMagic Hemalurgy]] does not require fuel, but it does require leeching abilities from Allomancers and Feruchemists [[spoiler: by stabbing them with particular metals and then permanently implanting them in oneself in [[EyeScream particular places]]]], and often leads to [[WithGreatPowerComesGreatInsanity great insanity]].
* In ''Literature/TheWayOfKings'' there are at least two types of magic falling under this law:
** Surgebinding produces various effects (from flight to trasmutation to healing), consuming the stormlight infused in gems in exchange
** The Old Magic is granted by a spirit called the Nightwatcher. Everyone who deals with the Nightwatcher can ask for a blessing, but in exchange must also receive a curse, chosen by the spirit. One minor character mentions trying to word his wish in such a way to avoid a curse, but is informed it doesn't work like that. You tell the Nightwatcher what you want, she considers it, and gives you a curse she finds to be of equal value. It might be related, but it could just as easily not.
* The entire basis of DevonMonk's ''Literature/AllieBeckstrom'' novels. Magic always exacts a price, usually in pain. The user can choose between an hour-long migraine or a week-long head cold, but the price will be paid. For the protagonist, magic also takes her memories. For small spells, she'll forget what her stepmother's name is. For large spells, she'll wake up with a three week hole in her life. She carries a journal with her everywhere, with her name, address, etc. written on the first page, just in case.
* In the ''Literature/BrasAndBroomsticks'' series by Sarah Mlynowski, if someone uses magic to get something, that thing will be taken from wherever it comes from. For example, the main character's sister makes oranges to give to the homeless, but there is an orange shortage in stores in the area.
* At the end of ''[[DuelOfSorcery Changer's Moon]]'', [[spoiler:[[MagicKnight Serroi]] turns [[EvilSorcerer Ser Noris]] [[TakenForGranite into a tree]].]] The price? [[spoiler:She turns into a tree herself]]. (Although that was less a function of the magic itself—she'd previously [[spoiler:turned {{mooks}} into trees with no ill effect]]—than of using it on an opponent of such power.)
* Roger Zelazny's ''Changeling'' follows this logic for moving items between universes.
* Stephen Donaldson's second ''ChroniclesOfThomasCovenant'' has the mere mortals of the land required to shed blood (both their own and the blood of others is used) in order to manipulate the Sunbane.
** The first chronicles has Mhoram realizing [[spoiler:the key to power is despair, which will lead to the destruction of the Land.]]
* In ChinaMieville's ''Literature/IronCouncil'', a character is beholden to an interesting version of this. They are a monk of the God of Secrets; part of this means that they can ask the god for knowledge about a secret: a secret path, what the enemy is planning, and so on. But they have to give up some of their own knowledge to do so. The character in question lost knowledge of her gender as a result of this.
* Sympathy in ''TheKingkillerChronicle'' follows the actual First and Second Laws of Thermodynamics, so that heat and work applied to a target have to be taken from somewhere and transfer of energy can be rather inefficient. In practice, this means that people have killed themselves trying to raise winds with their diaphragms and light candles with their body heat.
* In the Creator/StephenKing novella ''[[Literature/FullDarkNoStars Fair Extension]]'', [[LouisCypher George]] [[SignificantAnagram Elvid]] [[DealWithTheDevil offers to cure Dave Streeter's cancer]] at the cost of inflicting it on someone else. Since that someone else happens to be his ex-girlfriend who cheated on him with his ex-best friend, Streeter isn't exactly bothered.
* In Ian Irvine's ''Literature/TheThreeWorldsCycle'', use of magic (or "the Secret Art") is balanced by "aftersickness": debilitating headaches and nausea. Unskilled "mancers" may even get the aftersickness without the magical effect they were going for. This, however, is nothing compared to the risks of trying to draw too much power from [[FunctionalMagic the field]]: you get "anthracised", where the sheer power you're trying to channel burns you alive from the inside out. Of course, it's lovingly described in detail in one of the books.
* ''Literature/VorkosiganSaga: A Civil Campaign''; Cordelia Naismith-Vorkosigan points out that her home planet of Beta Colony is extremely sexually liberal, yes, but at the cost of being ''reproductively'' conservative. All women, by law, have a contraceptive implant, and people wanting to become parents are subject to a battery of tests before being licensed. Very few families are allowed more than two kids. This is not something the writer just made up, mind you; both the sexual liberality and the implants had been mentioned throughout the series, but it is the first time the connection was explicitly explained.
* ''Literature/TheWheelOfTime'': While use of saidar is more or less free, the Aelfinn and Eelfinn operate on this basis, and from most non-main characters important things like valuable knowledge, cooperation and items must be bargained for, often with extensive negotiations over the price the heroes must pay.
* In Rudy Rucker's Master of Space and Time, time travel [[spoiler:supposedly]] requires sending something forward in time if you send something backward. The twist is that the author ascribed to the shrinking universe theory, which mean that sending his pet lizard forward in time resulted in Godzilla: Jersey Shore.
* In ''Literature/{{Coda}}'', one year powering the city takes a year off your lifespan. This is what killed Haven's parents.
* In the sequel to ''Literature/ThoseThatWake'', healing someone via the neuropleth can bring them back from the brink of death, but it erases you from existence.
[[/folder]]

[[folder:Live Action TV]]
* Ben's healing talent in ''Series/{{Carnivale}}'' worked by drawing LifeEnergy out of the surrounding area. Cure a little girl of polio, the crops wither as she skips away through the cornfield. Heal a broken arm, a bunch of fish go belly-up in a nearby pond.
* Chloe Sullivan on Series/{{Smallville}} got a similar power in the sixth season finale (in this case, she died and came back to life). In the Seventh season, it is explained that she can heal non-fatal wounds so long as she herself takes on that wound (i.e. to heal a paper cut on Jimmy Olsen's finger, her body compensates by receiving a wound of similar size on the same spot on her body).
* One episode of ''Series/ForeverKnight'' featured a mystic healer that could take darkness out of people. However, said mystic happened to be a novice at her craft, and didn't know that this darkness had to be put somewhere, (usually into an inanimate object of some sort), and wound up absorbing it herself and being overwhelmed by it. The episode had a really sad end to it, [[spoiler: Nick was quite close to becoming human again, with most of his vampiric urges gone. But she herself was absorbing his darkness and becoming a vampire. She died from "OD'ing" on his evil, which he re-absorbed into himself.]] Her grandfather alluded that she might have been capable of fully healing Nick (or at least making his gains permanent) if she had been more skilled.
* Ned's talent on ''PushingDaisies'' works the same way, by killing one thing of equal magnitude to whatever was brought back from the dead if the dead thing's alive for longer than 60 seconds. So, if a person's brought back from the dead, then another person is going to die to keep them alive. (See also: BalancingDeathsBooks)
* ''QuantumLeap'': Every time Sam leaps, the person he's replaced ends up in 1999.
* In ''Series/BuffyTheVampireSlayer'', when Willow sends Buffy back in time to meet the sages who created the first Slayer, a demon is brought forwards to take her place. Willow says that this is to avoid violating the First Law of Thermodynamics.
** Similarly, though perhaps a more accurate example of this trope, when Buffy is brought back to life at the start of Season 6, the spell creates a brand new demon through a process Willow calls "Thaumogenesis". As she explains, the universe doesn't like you getting things for free, there has to be a cost and they asked for a huge gift; Buffys life back, so the universe said "Ok, but you have to have this evil demon too". This fits the trope as its a Good Thing/Bad Thing equal parts deal but as Anya rightfully points out "That's not a cost, that's a gift with purchase".
*** And to get the ingredient [[BloodMagic (blood)]] that Willow needs to cast the spell to bring Buffy back, Willow stabs a cute little fawn with a knife.
*** Same thing happened with Jonathan in "Superstar," although in that case the existence of the demon was required for the spell to keep working.
**** The demon brought forth to take Buffy's place happens once again in the Season 8 comic when Buffy time travels.
* In ''Series/{{Torchwood}}'', the [[CameBackWrong Resurrection Gauntlet]] in the first series could bring someone back from the dead, but usually only for about one or two minutes. However, with enough empathy from the gauntlet's user, a person could be brought back completely from the dead - but the person who used the glove would slowly give up their life (including any fatal wounds the formerly deceased had) while the previously dead person would get healthier and healthier.
** Of course, since Jack has unlimited life force, the whole thing [[GoneHorriblyRight gets turned on its head when he ressurects Owen]]
* Whenever Frank Parker steps back in time in ''Series/SevenDays'', his self at that time period vanishes from the time stream. It's explained that this is because the same set of molecules cannot exist in two places at the same time.
** This is the sort of thing that makes sense in magic, but not if you try to science it up. His body is ''not'' the same set of molecules that it was last week, so the results ''should'' be a little bit messy. I'd expect that to maintain secrecy, he wouldn't be allowed to eat anything that hadn't been on the base for a week.
** This almost blows the secret once, when Parker steals something the first time through, when he's investigating the problem, and takes it back with him. So it 'magically' vanishes from villain out of air-tight security, out of a locked briefcase, and the villain twigs that that shouldn't be possible, and starts investigating this 'Backstep' project.
* In ''PowerRangers'' there are some examples of ''TheSixthRanger'' being limited due to their awesome power. The most notable were the Green Ranger and the Titanium Ranger. The Green Ranger’s powers were damaged, such that every time he used them, they would weaken, putting physical strain on himself. The Titanium Ranger was branded with a cursed tattoo of a cobra on his back. Every time he morphed, the cobra would move up a little. If it got to his neck, he would die.
* This was also used as a plot point in the BBC Series ''Series/{{Merlin}}''. [[spoiler: Arthur was conceived through the use of magic at the cost of his mother's life; she died during childbirth.]] There was even a reveal in this scene because [[spoiler: Nimueh, a recurring villainess in the series, was the one to use the spell that conceived Arthur in order to grant Uther an heir by his barren wife. She herself knew there would be consequences of this spell, but she didn't know how they would appear.]]
** This reappeared in the season finale, where [[spoiler: Merlin offered [[BalancingDeathsBooks his life in exchange]] for Arthur's, as the prince was being killed by an incurable poison. However, this didn't work as planned, as Nimueh took Merlin's mother's life instead, so his mentor/father figure Gaius offered up his life (confused yet?). The whole saga ended when Merlin killed Nimueh and used her life to save Gaius.]]
* In the ''Series/{{Supernatural}}'' episode "Faith", a Reaper can restore a dying person's life, but only at the cost of another's. The woman who was holding the RestrainingBolt uses this to set her husband up as a faith healer, while using the exchange to murder various of her faith's bugaboos. When the brothers break the leash, she learns the hard way that TrueNeutral [[EvilIsNotAToy is not a toy]].
** In the episode "Criss Angel Is a Douchebag", real magic is used to save a magician from lethal escape tricks. The cost of saving his life is that another person dies in the same way the magician would have been.
* In an episode of BigWolfOnCampus, Merton was given a magical watch that could turn back time. Unfortunately, every time he uses the watch, he loses some of his knowledge.
* This appears to be how the Phoenix Talisman works in ''Series/{{Warehouse 13}}'', though its behavior is not consistent. It will resurrect its user after his or her death, and then the trauma of that death will be transmitted to a random person in proximity. However, it's not always a one-to-one substitution; during Mac Pherson's demonstration of the Talisman, two random mooks die in exchange for the single demonstrator. It may be proportional to the amount of trauma the user suffers.
** Almost all the artifacts do something bad as well as something good; that's why they need to be collected in the Warehouse for safekeeping. Whenever the subject of using artifacts for good is brought up, this trope is the rebuttal--no matter how much an artifact seems to be helping people, it's also causing an equal amount of harm somewhere.
* In ''HouseOfAnubis'', the Cup of Ankh will grant immortality to the person who drinks the Elixir of Life out of it, but [[BalancingDeathsBooks someone else will die to replace the life that would otherwise have ended eventually.]]
* In ''Series/KaizokuSentaiGokaiger'', the [[SeriesGoal Greatest Treasure in the Universe]] has the [[RealityWarper power to change history and reality]], and confirms that it could be used to erase TheEmpire from history...but doing so would ''also'' [[RetGone retroactively erase]] all of the SuperSentai from existence. The Gokaiger end up deciding not to do this, since the Sentai mean too much to humanity and it isn't their right to make that call for the entire planet.
* Series/OnceUponATime apparently has it as one of its key tenets. "All Magic comes with a price." While Fairies don't include such a thing normally in their use of magic, it's possible that their conditions for magic use is the price others must pay. (either that or the dwarf mines are the price to make the magic work)
[[/folder]]

[[folder: Music]]
* Music/TheBeatles in "The End" from ''Abbey Road'': "And in the end, the love you take is equal to the love you make."
[[/folder]]

[[folder: Mythology & Religion]]
* The concept of [[CallItKarma Karma]] in many religions.
* In {{Euripides}}'s ''Theatre/{{Alcestis}}'', Admetus' wife, Alcestis, offered to [[BalancingDeathsBooks die in his place]]. And then [[DeusExMachina Heracles showed up]] and ''[[DidYouJustPunchOutCthulhu beat up]]'' ''[[TheGrimReaper Death]]'' to save her so that there could be a happy ending. Honestly, didn't these Greeks read Orpheus?
[[/folder]]

[[folder:Tabletop Games]]
* In ''TabletopGame/BattleSpirits'', everything has a cost of core, even if it's just a low amount, though it can be reduced through the cards specificed reductions and other effects. Some double symbol spirits require the tribute of another spirit to summon them.
* The Defilers in the DarkSun ''TabletopGame/DungeonsAndDragons'' setting. Both Defilers and Preservers need to drain life forces in order to power arcane magic, but defilers drain it away without worrying about the consequences, while Preservers are careful not to drain enough to kill the plants and animals nearby that are contributing.
** Considering that Dark Sun is a gigantic desert world, it's not hard to deduce the popularity of defiler magic.
* In the {{Ravenloft}} setting, curses can be invoked by ordinary people, but attempting to do so invites a Powers check. If failed, the curse-layer will suffer karmic retribution from the Dark Powers. In an Equivalent Exchange Of Payback, the curse is actually more likely to work if the curse-layer fails this check.
* Most magick in ''TabletopGame/UnknownArmies'' works this way. The "value" of certain actions varies based on what kind of adept you are. Typically an adept gets a "minor charge" for some kind of ritual that's easy enough to be performed every day or so, a "significant charge" for doing something very difficult and painful, and a "major charge" for doing something nigh-impossible. And even the magick that seems to be free usually isn't. In TabletopGame/UnknownArmies, ain't nothin' come for free.
** There's even an in-game term for the concept in UA: The Law of Transaction.
* The ''TabletopGame/VampireTheMasquerade'' version of the Tremere House and Clan is practically built around this trope. Originally mages, the Tremere turned themselves into magical creatures when magic began weakening, out of a supposition that said creatures would last a while longer during magic's decline. However, in doing so, they lost the essence that made them mages in the first place, and got what was, in essence, a surrogate. The price of power, indeed.
* Present in the TCG TabletopGame/MagicTheGathering to an extent. Every spell has a cost. Most are simply {{Mana}} drawn from the land, but others require a life (yours or your creatures), the land itself, or even time (skipping a turn). Several cards will actually kill you if the cost is too much for you to afford.
** Perhaps the most famous example of this is Necropotence. You don't draw normally near the beginning of your turn; instead, you may pay X life to draw X cards at the end of your turn. However, this particular exchange turned out not too equivalent: both cards with this effect proved to be broken beyond imagination.
*** For the uninitiated: Unless you know exactly what you are doing and have built your deck specifically to respond to various threats, playing Necropotence is pretty much suicide.
* In ''TabletopGame/{{Warhammer}} Fantasy'', the species known as the Dragon Ogres succeeded at making a deal with Tzeentch that rendered their entire species immortal and able to subsist on lightning alone as energy -- but the spell also struck them all sterile, so no new Dragon Ogres has been born since. [[TheChessmaster Tzeentch is kind of a dick like that]].
* In ''TabletopGame/GeistTheSinEaters'', a Sin-Eater can come back from the dead easily (yes, [[FirstEpisodeResurrection more than once]]). Problem is, in addition to the act knocking a chunk off your KarmaMeter, your [[OurGhostsAreDifferent geist]] is going to draw that life force directly from someone else. And when you wake from your brief dirt nap, your face is covered in an ectoplasmic caul that conveys ''all'' the details of that person's death.
* The magic system in "The Valdorian Age" (a setting for [[HeroSystem Fantasy Hero]]) can be summarized as "you convince otherworldly/extraplanar beings to do something for you". However, they're doing it as a favor, and eventually they will require a favor from you in exchange ... which, depending on how big your debt is when it gets called in, could involve things like killing all the inhabitants of a village in one night. No one ever claimed those otherworldly beings were ''nice''.
* Several of the magic systems in ''LegendOfTheFiveRings'' work this way. Shugenja do not so much manipulate the elements themselves as convince elemental spirits to do them favors, and much of their duties involve making sure said spirits are happy. ''[[BlackMagic Maho]]'' is directly powered by the shedding of blood - but good news! It doesn't have to be ''your'' blood.
* In ''[[ChangelingTheLost Changeling: the Lost]]'', this is the basis of Pledgecraft: Changelings can create magical bargains known as Pledges, each incorporating a task, a boon, a sanction and a duration. Boons and durations count toward the cost of a pledge, while task and sanction count against it, and all Pledges must have a net worth of zero. A clever changeling can still game the system a bit by such expedients as making a pledge that if given a healer's knowledge for a day, they will use it at least once that day, on pain of misfortune. If you make the pledge when a companion is already injured and needs first aid, satisfying the task is laughably easy.
* In ''TabletopGames/{{Exalted}}'', the Sidereal Exalted have the charm called Of Things Desired and Feared, which uses astrology to provide an infaillible way to achieve ''any'' objective you can state... for an appropriate price. If what you want to achieve is small, it is likely the price will be something you are willing to pay (a few bruises, the lost of an item, etc.). But if you ask for a Fate-proven way to defeat BigBad or otherway break the game, the price will be something like "your life, the lives of everyone in your Circle, and the complete destruction of all the thing you value and care for".
[[/folder]]

[[folder:Video Games]]
* The ''VideoGame/MonkeyIsland'' games have this as an element of Voodoo magic. In ''VideoGame/TheCurseOfMonkeyIsland'', Guybrush needs to find need a diamond as big as or bigger than the one that turned his fiancé to stone. In ''VideoGame/EscapeFromMonkeyIsland'', he needs to make an Ultimate Insult talisman as large as or bigger than the original in order to counter its effects. (No explanation is given on why his enemies can counter Guybrush’s insults even when he has the larger talisman, although it ''is'' only the ''second'' biggest monkey head Guybrush has ever seen...)
* ''VideoGame/OracleOfTao'' has a variety of these. Most magic uses MP as its price, but some abilities are CastFromHitPoints. On the other end of the scale, you have a ManaShield. And then there's Elias's alchemy, which aside from making cool items, has certain alchemy spells. But in order to learn each spell or make a super-rare item (like gold), you sacrifice anywhere from a level to 10 levels (and you can combine spells together, meaning there's a chance you might end up making the same spell more than two times).
* Whenever the Nameless One from ''PlanescapeTorment'' [[spoiler:dies and comes back, someone else somewhere on the Great Wheel dies as a result, becoming a tormented shadow whose only desire is to hunt down the Nameless One and kill him again]].
* In ''VideoGame/ValkyrieProfile'', the ritual of Soul Exchange allows someone to sacrifice their own life to bring someone else back. However, it won't work on someone who died by using the same ritual, and one character ends up undead for attempting this.
* This was a main part of ''VideoGame/TheImmortal''. Your character starts with a magic amulet that, you eventually discover, has the power to kill a dragon (the creature that awaits you at the end of your adventure). However, it turns out the amulet also kills the person who uses it (you could easily discover this at any point by using the damned thing), and the whole game is an EvilPlan by the BigBad to get you to kill the dragon (sacrificing your life in the process) because he needed the dragon dead, but obviously couldn't use the amulet himself without dying.
* In ''VideoGame/TheLegendOfZeldaTwilightPrincess'', Link can acquire a magical suit of golden armor which will prevent him from taking any damage. However, whenever he wears it, his wallet is slowly drained of money, and he loses extra money every time he takes a hit. Once he runs out of Rupees (the coin of the realm), the armor weighs him down considerably, making his movements slow and awkward.
* A popular mod for ''{{VideoGame/Minecraft}}'' by the same name revolves around this trope. This mod gives every item in the game an EMC value (Energy-Matter-Currency). These items can be exchanged for each other using a Minium Stone/Philosopher's Stone if the EMC value is the same.
* The world ''[[VideoGame/KartiaTheWordOfFate Kartia]]'' revolves around magical cards, Kartia, that can be used to create basically anything, from living battle-creatures to spells to specific kinds of food and drink. There are a number of Original Kartia (such as "Life" "Death" and "Human"), the use of which is forbidden and will automatically cause the death of the caster. For most of the game, the player and the characters believe that the Kartia creates things from nothingness [[spoiler: but as it turns out, it just takes things from Eden, a parallel world inhabited by Elves and rich with natural magic. In fact, it is theoretically possible to "create" the whole Eden with Original Kartia, something that is half-accomplished twice during the story. It also turns out that Original Kartia doesn't kill the user, but rather transports them to Eden.]]
* ''VideoGame/WorldOfWarcraft''
** Warlock magic is said to work like this.
-->''"With hell's fire, you make a bargain. It costs a little of yourself. The warlock's way was quicker, more effective, or so it seemed. But there comes a time when a price must be paid, and sometimes, it is dear indeed."''
** Don't expect to ever have to pay that price [[GameplayAndStorySegregation yourself]] though.
** Examples include the Life Tap spell (converts health into mana points), Health Funnel (heals your pet demon at a cost of your own health), Hellfire (which burns you as well as your enemies), and Ritual of Doom (summons a powerful demon; originally one party member would die upon completion, now one member is simply badly wounded). Other spells are fuelled with Soul Shards, gained by killing foes with a particular spell.
*** Although now, Warlocks have a low-level spell that lets them restore all their soul shards by apparently breaking off little pieces of their OWN SOUL (at no real cost in gameplay terms, though it is interruptable and thus best used out of combat/between fights).
*** Doomguards don't even require the ritual of summoning or sacrifice of health anymore. Just hit a button, and BOOM he's there fighting on your side for 45 seconds.
** In the ''Ashbringer'' comic, Darion Mograine stabs himself with the title sword to free his father Alexandros's soul from the Scourge's hold, becoming a Death Knight himself. "I love you, Dad..."
* ''VideoGame/{{Persona 3}}''
** [[spoiler:Chidori sacrifices herself]] to revive [[spoiler:Junpei]] under this principle.
** From the same game, the skill Recarmdra fully heals all your allies' HP in exchange for yours becoming 1. It costs only 1% of your total SP, but it is still terribly risky given how [[WeCannotGoOnWithoutYou it's game over if the leader is KO'd]].
** Recarmdra is present in many games in the ''ShinMegamiTensei'' series, where it often does kill the user to fully heal the rest of the party. Because of [[WeCannotGoOnWithoutYou how the games are in general]], only the recruitable demons and not the main character can get it.
* The various Dark Powersets from ''CityOfHeroes'' tend to work like this, Dark Miasma in particular. Dark Miasma has some highly potent healing spells (Including the only rezz in the game capable of reviving multiple people.) but in order to use them you have to tap into the life force of your enemies. This is just icing on the cake, really...
** The only real downside of these powers was that you had to have an enemy to engage to use them. So you couldn't heal or rezz BETWEEN fights.
* In ''GuildWars'', many [[{{Necromancy}} Necromancer]] spells, mostly under the BloodMagic attribute, require the Necromancer to sacrifice a certain percentage of their maximum health, in addition to a small amount of MP, to cast. In addition, Necromancers also have many spells that require there to be a fresh corpse somewhere nearby for them to "exploit". These spells include raising undead servants, creating a "well" of energy that performs various effects, [[ImAHumanitarian healing]], or simply making the corpse [[{{Squick}} explode]].
* In ''Franchise/DragonAge'', Mages can perform magic effortlessly, but more powerful spells and rituals occasionally require the use of [[GreenRocks Lyrium]], distilled into a consumable liquid form for this purpose. [[BloodMagic Blood Mages]] however, can use either their LifeEnergy or another's to perform powerful feats that non-Blood Mages cannot, such as MindControl, as well as ripping the blood out of their opponents pores.
** While one can learn the skills safely through books written on the subject, these are usually restricted for obvious reasons, thus forcing more desperate (or foolhardy) mages to make a bargain with a Demon to gain the power. What the bargain requires changes from person to person, but most cases end with [[GrandTheftMe the Demon possessing the Mage]] and turning them into [[AndIMustScream an Abomination]].
* ''PhantasyStarIII'' allows you to visit a "technique distribution" shop to alter the potency of a magic-wielding character's techniques. With the use of a square-shaped grid, at the expense of one, another can be strengthened. In practice, thanks to {{Useless Useful Spell}}s, you'll usually end up maxing out Gires and utterly bottoming out Rever or Anti, since those had a high probability of failing anyway.
* In ''VideoGame/QuestForGloryI'', "Every curse has an equal and opposite countercurse."
* Equivalent Exchange is one of the major rules of magic in the {{Nasuverse}} both in the practical (a spell needs to take its energy from somewhere else) and the social sense (a magus will never do someone a favor without expecting something in return).
* The "Pandora" feature of ''VideoGame/StreetFighterXTekken'' allows you to sacrifice your partner to give your primary character eight seconds of unlimited super meter and amped-up strength. To make this even more of a desperation move, if those eight seconds elapse before you defeat your opponent, you lose.
* In ''FinalFantasyCrystalChroniclesRingOfFates'' the main characters Yuri and Chelinka has access to enough magic power for a CosmicRetcon, if they need to. However, using that power needs something in return. Chelinka pays by losing her soul for a longer period of time, Yuri, pays with part of his life.
* In ''FinalFantasyXIIRevenantWings'' summoning Yarhi siphons part of your Anima, the part of you that feels, which is why all the Aegyl who regularily use Yarhi to protect them selves for random monsters appear rather emotionless. [[spoiler:Their "god" draining their Anima for himself is a bigger factor in this though.]]
* While Calypso from ''VideoGame/TwistedMetal'' is either a LiteralGenie or a JerkassGenie, the ending for his daughter winds up universally balancing it out, by wishing that [[spoiler: the car crash that killed her mother never happened, the universe balanced things by making the daughter get in the crash herself and put into a coma she probably wouldn't wake up from.]] Even Calypso is genuinely heartbroken about this outcome.
* In ''VideoGame/JunipersKnot'', the rule of [[spoiler:the spell binding the fiend]] is of this type: [[spoiler:a life for a life.]]
[[/folder]]

[[folder:Web Original]]
* The perfume in ''ErikasNewPerfume'' seems to function like this, though given she got FakeMemories out of the deal it's debatable if Sarah gave up anything.
* NewYorkMagician: Michel has to use energy from things like fired bullets and flashbulb capacitors to power his magic. One favorite trick is to fire one bullet, and use the energy from that to do magic to the next one he fires. [[spoiler:Then there's the automatic summoning spell running off an old IRT substation.]]
* In the AxisPowersHetalia fan game HetaOni, England reveals that he can destroy the Grays all on his own, using his magic. However, [[spoiler: it comes at the cost of his eyesight.]]
* Fey of the WhateleyUniverse is a massively powerful mage who can, among other things, pull magical energy from ley lines. It turns out there's living things at the other end of those ley lines, and over the course of the first year she unknowingly caused several ''ecological disasters''.
* Wiki/SCPFoundation: This is [[http://www.scp-wiki.net/scp-738 SCP-738]]'s deal. If you sit in the chair across from him, he will make various offers to you. Alternately, you may request something from him. He will grant this request, provided it is a request ''for you'' (you cannot request something on someone else's behalf). However, the price is that an equal amount of misery will be introduced into your life as the amount of joy this request brings.
[[/folder]]

[[folder:Western Animation]]
* Eric gets a lecture to this effect from Dungeon Master in the "Day of the Dungeonmaster" episode of ''WesternAnimation/DungeonsAndDragons''.
* ''WesternAnimation/{{Futurama}}'' uses this trope when regarding time travel in ''Bender's Big Score'' due to the discovery of a paradox-correcting timecode. The timecode ignores all of the problems that normally arise with time travel as long as the end result is close enough to the main timeline. If there are multiple copies of someone created by time travel, then fate will make those copies exceedingly doomed until all but one are dead and the universe considers that perfectly fine. Somewhat justified as the code functions by drawing time bubbles from a creature previously implied to be God, who assumably would be immune to paradoxes.
** And then [[spoiler: Bender [[NiceJobBreakingItHero makes an uncorrigible paradox]].]]
* The fact that all magic has a price is used repeatedly in the {{DCAU}}. In one notable case in ''JusticeLeagueUnlimited'', Circe's price for releasing a BalefulPolymorph curse she had on Diana is something from Batman he can never regain once lost... ''[[http://www.youtube.com/watch?v=uoRundeX6O0&feature=related his dignity]]''.
** It was still a CrowningMomentOfAwesome, though. Because even when giving up his dignity, Batman's that much of a BadAss.
** Given there didn't seem to be any cost associated with turning WonderWoman ''into'' a pig (or most of the other magic on display) this may have been less about the magic itself having a price, and more about Circe not wanting to undo the spell without getting ''something'' out of it.
* ''WesternAnimation/KimPossible'' has Monkey Fist, who agreed to walk the path of the Yono in exchange for the Yono's power. It was granted to him...until he lost. [[spoiler:Then he followed the path as agreed, [[PutOnABusToHell petrification being the result]]]].
* The ending of ''WesternAnimation/{{X-Men}}'' take on TheDarkPhoenixSaga has Jean Grey [[HeroicSacrifice dying to stop the Phoenix threat]] as per source, but since here it was the real Jean posessed by a cosmic force, instead of [[TomatoSurprise the force itself taking her form]], she really does die. The force itself is okay though, but it [[MyGodWhatHaveIDone realises that it was wrong]], and [[MustMakeAmends offers to ressurect Jean]], requiring someone sacrificing their own LifeEnergy. Cyclops and Wolverine have a MoreExpendableThanYou moment before Phoenix informs them that the necessary amount of life energy can be obtained from several donors, without anybody dying.
* In ''WesternAnimation/BarbieInAMermaidTale2'', in order to exit one of Eris's inescapable whirlpools, someone else has to enter it.
[[/folder]]

[[folder:Real Life]]
* As seen in the description, the First Law of Thermodynamics makes this trope not just OlderThanDirt, but Older Than Everything But The Universe Itself. The ''second'' law of thermodynamics states that you can't really get even equivalent exchange -- you'll always "lose"[[note]]In scare quotes because the energy still ''exists'', it's just not doing anything useful.[[/note]] some of the input energy to waste heat.
** [[http://en.wikipedia.org/wiki/Noether%27s_Theorem Noether's Theorem]] shows that there's a one-to-one correspondence between conservation laws and physical symmetries. Conservation of energy is due to time-invariance, so as long as physical constants don't change, it will exist.
** Technically, increasing entropy is a strict conservation law. If you kept track of all possible outputs for a given set of inputs, the total entropy would remain constant. It's just that it's effectively impossible to keep track of it all, so you have some set of "possible" outputs that includes the real set, and has higher entropy.
* Most of chemistry is this. It's even been worked out mathematically, much to the annoyance of undergraduate chemistry students and those grading their papers.
** 6 [=CO2=] + 6 [=H2O=] + Energy -> C6-H12-O6 + 6 O2 (6 Carbon Dioxide + 6 Water + Energy -> 1 Sugar + 6 Oxygen)
*** And [[BunnyEarsLawyer RuBisCO]], which sucks at its job as a catalyst of that overall reaction, but is the only one that works.
*** C6-H12-O6 + 6O2 -> 6 [=CO2=] + 6H20 + Heat. [[{{QI}} Explosion in a custard factory]].
* TheGoldenRule can be interpreted as an attempt to apply this law of physics to justice and ethics.
* Also on a physical level, athletes, especially Olympic-level. They can do what seem like superhuman feats of strength, agility, endurance, etc., but the cost is devoting massive amounts of time training for a specific event, and burn out later in life.
** More generally, while any human is capable of performing similarly superhuman feats, untrained Muggles can only do so under great stress and at the cost of shutting down certain vital functions temporarily, as well as directly damaging their bodies.
* In a meta example partially related to the above, to achieve anything in life it is necessary for certain things to be used to reach this point. To breathe you must expend the energy needed to inhale oxygen into your lungs, to become successful at something (without relying on luck->chaos->entropy) you must devote a large amount of time to study your chosen field et cetera.
[[/folder]]

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