This could be something that you bought for a higher price than what most people would guess based on the item, or it could be something you bought for a normal price that has gained significant value as time has gone on.
What made me think of this question is a LEGO minifigure I got with my “The Hobbit: An Unexpected Journey” disc. It is Bilbo Baggins in a blue coat that was apparently only sold in that movie box only at Target stores. Even considering the exclusivity, I would have guessed maybe $10-20 for such a tiny piece of plastic, but there are sold listings on eBay from $80 to $225. I could possibly even get towards the higher end of that number since I still have everything in the original box in good condition. It’s not worth a ton compared to some other items people may own, but I think most people would not expect nearly that amount.
That’s awesome! I’ve collected a few coins but none worth that. I only have one ancient coin of the Gallic Emperor Tetricus II that’s in rough condition, but I only purchased it for $10. The only surprising coin I have is an 1809 US Capped Bust Half Dollar that is apparently a rare variant with some markings along the edge, and PCGS says there are only an estimated 900 of that variant that are still around. Unfortunately, there was already a deep scratch on the front when I bought it that likely lowers the value to that of the normal variant, but I still love the history and rarity of it.