Basic Trope: A powerful, important item is found in an unlikely location, its value seemingly unknown before the protoganist finds it.
Straight: While browsing a merchant's wares, Miki finds a dull, tarnished blade which is being offered at a very low price. This blade turns out to be an ancient weapon capable of channeling electricity, making it far stronger than it appears to be at first glance and an excellent weapon for her.
Exaggerated: The blade is a legendary artifact reputed to be one of the only things capable of harming the otherwise invincible Emperor Evulz. Miki found it in a landfill.
Downplayed: The blade is just a toy, but it turns out to be a modestly valuable collectible.
The merchant has no way of knowing the importance of this particular weapon; it looks cheap, so he's selling it for cheap.
The sword performs its power only in the hand of a specific type of wielder or target. If no one of pure heart used it, much less against something or someone irredeemably evil then they wouldn't know about its holy powers.
The merchant boasts that he's got an Infinity+1 Sword in stock, but is actually blatantly lying to snare himself a few suckers.
Bob finds himself in an Aladdin's Cave full of wondrous artefacts. He drags out a bejewelled cup the size of his head... only to find out later that it's an entirely mundane, gold-plated pewter cup covered in glass gems, whereas everything else in there was magical.
A museum has a jeweled amulet on display that is legendary for being the source of a local hero's power. Emperor Evulz sends some mooks out to steal it, and they do. Evulz finds out that the "jeweled amulet" was actually the hero's Magic Feather. It's technically the source of the hero's power (it was given to him by his Trickster Mentor to "give him strength and courage"), but doesn't actually do anything, being nothing but a piece of cheap glass on some thin rope.
Subverted: When Miki finds the blade, the merchant notices her interest and starts talking about how wonderful and important and pricy it really is.
Double Subverted: However, he's exaggerating how valuable he thinks it is to play her for a fool; when she balks at paying such a hefty fee, he admits that it's "just a beat-up piece of junk" and sells it to her for a very low price.
Parodied: Miki finds the blade in a barrel labeled "Discounted MacGuffins - Buy one, get one free!"
Zig Zagged: Miki finds a beat-up blade at a bargain price; it turns out to be a powerful weapon perfectly suited for her skills... but needs to be fixed up first, so she has to find somebody who can identify it and give it the care she needs. The first blacksmith she talks tries to play her for a sap and charge an insane amount of money for the repairs; he doesn't actually realize what it is, though, and just assumes she's an easy mark. The second smithy recognizes its value, but tries to force her to pay through the nose for repairs and steal the sword so he can sell it for an even higher price. Thankfully, the third blacksmith Miki tries recognizes its value and strikes a fair deal with her to fix it up, taking her on as an assistant and becoming her Mentor.
Averted: The merchant knows what the blade is, and charges an appropriate price for it.
Enforced: "Anyone who knows what the MacGuffin does would surely hoard it, but Miki has to get her hands on it somehow. Let's give the merchant the Idiot Ball, so he'll practically give it to her free."
Lampshaded: "Oh, yeah, this awesome electric blade? Some moron sold it to me for two bucks!"
Invoked: Penniless Miki goes looking in garage sales and secondhand shops specifically hoping to find something this awesome.
Defied: The merchant explores every acquisition he makes, so he can charge a price that reflects its awesomeness.
Discussed: "Be sure to check the pawn shop."
Conversed: "Ever noticed how these things always turn up in some dumbass mechant's stall?"
Since its value isn't immediately apparent, Miki chooses not to buy the beat-up blade and to save up for a normal, perfectly serviceable sword, unaware that she's passing up a great bargain.
Alternatively: Because the previous owner thought it was worthless, it is in terrible condition.
After the third can of sealed evil gets sold to a clueless muggle, the ancient conspiracy establishes a task force to inspect all pawn shops and garage sales. They pick up particularly valuable macguffins while they're at it.
Reconstructed: Though it doesn't look like anything special, Miki still needs a cheap start-up weapon, so she goes ahead and makes the purchase, discovering its real value afterwards.
Played For Laughs: Miki searches all over the city for the blade. After a long, tiring day of looking, her friend calls her up and says there's something she might like in the pawn shop.
Played For Drama: When Miki loses her sword, it ends up being sold for cheap again at the stall where she bought it. When she gets there, she finds out her Rival has purchased it just moments ago.