"They're never curing AIDS. There's no money in the cure. The money's in the medicine. That's how you get paid, on the comeback. That's how a drug dealer makes money, on the comeback...You think they're gonna cure AIDS? They're still mad about all the money they lost on Polio!" —Chris Rock
In Busou Renkin, Papillon tries a variation on this. Having planted a Homunculus embryo in Tokiko, when Kazuki confronts him, Papillon offers a cure in exchange for Kazuki's Kakugane. This could also overlap a bit with an unintentional Xanatos Gambit, given that the cure is a useless fake - if Kazuki makes the trade, then Papillon gets to study Kakugane technology and he still gets to turn one of his greatest enemies (at the time) into a minion. If not, then he loses out on the Kakugane... but still gets Tokiko as a minion. However, because the Kakugane is what brought Kazuki Back from the Dead, he can't remove it or he'll die. Kazuki can't make the trade and accidentally knocks out Papillon with one punch, at which point Tokiko arrives and tells him that the cure was fake.
In the Punisher MAX storyline "6 Hours to Kill", Frank is blackmailed with a cure to assassinate someone. He instead decides to take down as many criminals as possible before dying.
German comic Fix und Foxi had the (mostly) Lovable Rogue Lupo pull this off once. First, he let loose a lot of moths and sold people a spray against them. From said spray, people got an allergic reaction and had to sneeze all the time. He sold them a cure - which made people unbelievable thirsty. Fortunately he sold them a special lemonade which would cure their thirst - but totally mess up their hair. Then he sold them wigs. Which were badly made, with the hair falling out. Then finally, they got it. Cue the mob.
A heroic trickster variation. In an issue of Marvel Star Wars, Luke told an imperial officer he had poisoned him, and would give him the antidote once he gave Luke access to computer records. Feeling stomach pains, the officer complied, and then Chewbacca knocked him out. Turns out Luke only put soap flakes in the officer's soup.
In Red Sonja the thought-to-be-deceased King of Zamora is revealed to have created a "plague" (actually a poison) that he gave to his enemies in order to trick Sonja's former companion Annisia into massacring entire cities, believing them to be infected, while he holds the antidote.
V for Vendetta: The Norsefire group get their position by spreading a plague through several areas, and blaming it on supposed captured terrorists, and its leader Sulter won the election by a landslide, the party then distributes a cure through a medical company they control.
In Escape from L.A., Snake is infected with the Plutoxin 7 virus, which will kill him in 10 hours unless he gets the President's daughter and the EMP satellite control device she stole. Subverted in that at the end, it's revealed that "Plutoxin 7" is merely a fast-acting, hard-hitting case of... the flu.
The villain of the Matt McColm action movie Body Armor creates nasty viruses and makes money off them by then selling the cure.
Emo Phillips told a joke in which as a child, he had a lemonade stand where the first glass was free and the second was $5. The second glass contained the antidote.
A group of tourists goes on a guided tour of a rubber factory. First they see the floor dedicated to baby pacifiers, then the floor dedicated to condoms. One tourist notices that one in ten condoms is removed from the assembly line, goes through a separate machine, then gets put back with the rest. On asking, he is told that the machine pokes a hole in the condoms that go through it. After all, they've got to sell pacifiers somehow.
In Lois McMaster Bujold's Vorkosigan series, Baron Fell of Jackson's Whole has this as the basis of most of his business, selling both traditional military weaponry and their defenses, as well as manufacturing chemical and biological weapons along with their cures.
A variation occurs in John Collier's famous story The Chaser: The Little Shop That Wasn't There Yesterday sells a Love Potion for a pittance which the owner strongly implies will turn the main character's beloved into a Love Freak. The antidote in this case is the "chaser" of the title, which is some sort of poison to "solve" that problem.
Thomas of Magnus, the hero of Sigmund Brouwer's Wings of Dawn, uses this — the price of being given regular doses of antidote is continued cooperation from the agents pursuing him, all of whom claim to be with the good guys and want him to join them. In reality, this is a Batman Gambit to weed out which side is lying; Knowing that the villains have fewer compunctions about fighting dirty and think they're smarter than he is, Thomas is slipping non-lethal doses into every meal and providing them with flavored water as the "antidote". When he "inadvertently" allows them enough information to determine the recipe of the supposed antidote, the villain works it out and seizes the opportunity to... poison himself. OOPS.
In Inheritance Cycle this gambit is used to ensure that Arya couldn't escape prison, as only her captors had the antidote.
The Corporations of Oryx and Crake, particularly HelthWyzer, made a business out of creating new diseases, inserting them into their vitamin supplements, and then selling the proprietary cures at high prices. This practice came back to bite the human race in the butt later, when Crake takes it to the next level.
In William Gibson's Neuromancer, protagonist Henry Case is controlled by his employer via sacs of poison placed into his blood vessels.
The villainous corporation in Confessions of Super-Mom makes both insulin and cereal, and deliberately uses the cereal to give children diabetes.
In Best Served Cold by Joe Abercrombie, master poisoner Castor Morveer and his apprentice Day use this a number of times. In some instances the trope is played straight, while in other cases there was actually no poison at all; in one of those cases, the proffered antidote is actually the real poison.
Fails epically in the first book of John Ringo and David Weber's Prince Roger series. Roger and his Marines are poisoned and ordered to fight for a tin-pot dictator with this gambit... which backfires amazingly because they're from a totally different planet. Even worse for the would-be poisoner, the poison used actually helps the humans, inspiring The Medic to check another poison for providing a mineral necessary for biological functions but unable to be manufactured by their food replicators, and finding that it can help stretch out the suplements required by the civilian team lacking the nannites doing the conversions.
Barbara Hambly's novel The Ladies of Mandrigyn pulls a particularly delicious version of this gambit: at the beginning of the book, a poisoner slips Sun Wolf a particularly dreadful poison and daily casts spells keeping it from affecting him, but will not remove it from his system until he's completed a task. Much later, he decides dying horribly is better than the alternatives, and escapes to crawl off and suffer the effects of the poison... which turn out to be the lost shamanic initiation everyone's been searching for the whole book. It's just better known as a poison, because it IS torturous and only a few are equipped to survive it.
In the Lionboy books, the Corporation induced an asthma epidemic in the general population in order to sell inhalers.
Artemis Fowl once tricked a sprite into drinking a bottle of Holy Water, then offered her a shot of magical springwater that would counteract the Holy Water's effects in exchange for the opportunity to study The Book (the fairy equivalent of the bible).
Seen a few times in Perry Rhodan, typically as a longer-term blackmail plot: apply a relatively subtle, slow-acting but deadly futuristic poison to the victim (or, for extra dog-kicking points, to somebody else the victim cares about), then regularly supply them with just enough counteragent to keep the poison in check but not actually neutralize it for good. The method has been used by planetary dictators, intelligence agencies of the more unscrupulous persuasion, and at least one starship captain using it to blackmail her own (admittedly likewise shady) crew; naturally, employing it is a pretty good sign of the perpetrator having crossed the moral event horizon some time ago.
Seen in Dune in a unique variant, directed at one person. When Thufir Hawat is captured by the Harkonnens, they administer a "latent" poison to him that is harmless on its own, but will remain in the body and turn poisonous when introduced to another otherwise-harmless substance. The Harkonnens make sure that Hawat knows how he has been poisoned and that they have the second half of the poison. They use this to blackmail him into working as their new Mentat after Piter De Vries is killed. As for why they'd do it this way, they know that Hawat is a remarkably suspicious man, and would take precautions to ensure that other, more obvious techniques to control him would not work (if threatened with physical violence, he might try to fight it off—or accept death. If threatened with poison, he'd get a taster. But a taster isn't susceptible to this poison...).
In Jhereg, Vlad and Kragar uncover an episode of this in the history of their target, Mellar: he won a noble's favor and aid by getting said noble in touch with a witch that could cure a plague. Of course, Mellar had hired the same witch to spread that plague to create that opportunity in the first place.
Elizabeth does this to get Jim out of prison in the season finale of Terra Nova. She was bluffing. The "cure" she injected was a sedative.
In Babylon 5, Edgars Industries creates a virus that kills only telepaths, but also make a cure. They intend to turn some into slaves and kill the rest. PsiCorps itself finds out about this and takes messy revenge. (They keep the cure.)
In another episode, a dual-latent-poison version (similar to the Dune example above) is used. When Lord Refa visits Londo on Babylon 5, Londo has a drink waiting for him. Londo asks Refa to end his association with Mr Morden and the Shadows, and when Refa asks why he would do that, Londo famously replies:
Londo: Because I have asked you; because your sense of duty to our people should override any personal ambition; and because I have poisoned your drink.
Then, Londo explains to Refa that the poison won't kill him right away, but rather would fester in his body and not do anything, but turn deadly when it met a second poison, which is also harmless on its own, and which Londo also had and was capable of administering (remember, having a taster won't help, since the taster won't be susceptible to the second poison). Refa complies. For a while.
The Americans: Philip and Elisabeth poison the son of the Secretary of Defense's maid in order to get her to steal a clock and then return it after they've planted a bug on it.
A variation in Merlin in that it's the hero who pulls it off, using a villain's ploy against them. On realizing that Morgana is the source of the spell that renders all of Camelot under an enchanted sleep, Merlin tricks her into drinking water spiked with hemlock. As she lays dying Morgause (who cast the spell in the first place) bursts in and Merlin breaks a deal with her: he'll give her the name of the poison if she lifts the spell over Camelot. She agrees in order to save Morgana's life (though if she hadn't, the spell would have been broken anyway with Morgana's death).
Almost Human has one episode involving an organ-trafficking scheme in which people desperately in need of new hearts are fitted with bio-mechanical hearts that are set to fail after thirty days. The timer can be reset, of course, for a very exorbitant fee.
Gogol's introductory episode in Nikita had the Russian paramilitary group capture Nikita and inject her with a Division-developed poison, with the antidote to be administered after she completed an assassination for them. Alex manages to smuggle the antidote out of Division for Nikita, who then ensures the assassination fails.
One of Chris Rock's bits on "Bigger And Blacker" accuses essentially the entirety of Western medicine of this, citing the fact that few diseases have been fully cured (as opposed to "patching it up" so that you can live long enough for them to "get more of your money") in recent years. One hopes that the Insane Troll Logic involved is merely Rule of Funny and that Mr. Rock is aware of the real reason (that the easy-to-cure diseases have already been cured, and what remains is more challenging).
[Insert disease here] is occasionally accused of having been intentionally created and introduced into the populace in various Urban Legends and conspiracy theories.
Used on a global scale in Deus Ex, with the synthetic disease "The Gray Death" (and very expensive vaccine "Ambrosia").
The original Baldur's Gate had a side-quest where you were poisoned by an assassin, so you'd die in 10 days if his partner in crime weren't ready to help you... for a price of removing the geas his "partner" put on him to make him cooperate.
Eidolons of Final Fantasy XIII start out by casting Doom on your party leader, leaving you with a time limit to defeat them. If you beat them, though, you get a fancy new summon!
The Joker does this in Batman: Arkham City, poisoning Batman with the same disease that's slowly killing him and making the Dark Knight find a cure. Batman merely responds that he's find with both of them dying, but the Joker anticipated that and managed to poison people all over Gotham with it, so now Batman really has to find a cure. Ultimately, Batman cures himself, Word of God is that he manages to save Gotham, but the Joker's actions to lead to him not getting the cure and so he dies.
Yo-Jin-Bo employs this trope, albeit mostly offscreen: in Bo and Ittosai's paths, the ninja Kasumimaru reveals that he persuaded Ittosai to turn mole by managing to cut him with a poisoned knife during battle, and then promising him money and the antidote in return for his help against the heroes. Interestingly, Kasumimaru apparently handed over the antidote as soon as the agreement was made, since it's never an issue even when Ittosai inevitably reneges on the deal.
As the plot of the Game ModMarathon Rubicon unfolds, the player can learn that this is The Plan of Dangi Corporation. Which of the game's endings you get hinges on whether the player does anything to stop it. Naturally, the ending where the player does nothing to stop it is not pleasant.
Touhou has a non-poison version. Mystia Lorelei uses her blindness-inducing Magic Music to sell grilled lamprey to humans as a blindness cure, with the intent of getting them so used to fish they stop eating poultry (she's a bird youkai).
The title character in Nick Chase and the Deadly Diamond was forced to steal the stone in order to receive the antidote to the toxin "Mr. X" had poisoned him with.
It's strongly implied that Ganon did this (releasing a plague into Hyrule, then arriving incognito as a sorcerer able to halt the plague) in the lead-up to A Link to the Past.
In Mega Man 10, Dr. Wily secretly spreads the Roboenza virus to afflict robots and leave humans helpless, then reveals that he caused it and says that anyone who wants to cure their robots should come work for him.
Played with in the Dead Rising franchise, as the company which holds the zombie plague medicine has to intentionally start outbreaks in order to create the medicine.
Snadhya'rune runs a nether cult that is obsessed with the idea of tainting, which basically involves infecting a Fae's soul with a demon, and as such has been spreading the taint far and wide. Getting tainted generally means being consumed by the demon, but Snadhya'rune has learned how to master control over it and is promoting it as a form of "enlightenment".
She threatens to annihilate all of the clans with a deadly infectious flower. Only those who bend to her rule will be given the cure. Surprisingly enough, a significant portion of the population strongly supports her goals. She has her first Villainous Breakdown in the entire strip when Sarghress assassins kill the scientist responsible for the "Cure" part of the gambit. Making sure another scientist can make the cure becomes a very high priority for her. Unfortunately, said scientist left No Plans, No Prototype, No Backup due to a desire to keep her work a secret. As a result, Snadhya's flower plague quickly spreads beyond the control of everyone, including her.
Given a Mad Scientist twist in Girl Genius when it turns out the poisoner is also the cure - as long as he's alive and close by there is nothing to fear from the poison.
Of course, he failed to properly take into account that the one poisoned was also a Mad Scientist. She couldn't cure herself, but she could make another creature have the same preserving effect as the poisoner.
In Garanos the Big Bad Gharsena is the one who made the disease Gailen is dying from, a fact she uses to force Gailen to be her mole. When Gailen figures it out she attempts to kill her, but Gharsena just magically accelerates the disease until it kills her.
One episode of Gargoyles double subverts this. Demona tries to pull a deal of this kind by shooting Elisa with a poisoned dart and coercing the gargoyles to help her, but unbeknownst to her Elisa's badge deflected the dart. The Gargoyles spring the trap anyway, out of curiosity, and so Demona will think she succeeded.
In the Drawn Together episode "The Other Cousin", Clara poisons Hero and promises to give him the antidote if he shows her cousin Bleh a good time. This becomes a What Happened to the Mouse? moment in the broadcast version when he's not seen getting the antidote in the episode (the extended DVD version does in fact show Clara giving it to him).
There was a thought experiment that played with this trope. The premise was that your best friend is dying of a unique disease, and the only cure is held by a doctor who wants more money for it than you can get. It's supposed to provoke questions of what morality truly means.
The S'Hamala or Chumash of California had antap, or medicine men/women, who poisoned enemies and then blackmailed them.