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.
The most valuable thing I own is my pokemon card collection. I was obsessed when I was kid, but unlike everyone, I kept going through Rocket, Gym heroes and challenge, all the way to Neo Destiny and Legendary Collection. Possibly the rarest cards I have are two 1ed foil discovery Tyranitars and one 1ed foil Blaine’s Charizard. My cards are very well loved and things have calmed down since the pandemic, but it was very weird, especially considering I got a booster box of Neo Discover for 50 bucks in 2009