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.
I have some rare coin collection that I inherited. When my great uncle passed away he just asked everyone to be fair when they go through his stuff. I told everyone they could go ahead I wasn’t too worried about getting anything of his as he didn’t really keep much sentimental stuff. My mom put a few things aside in a box that she thought I’d like. When I finally had time to go through everything I noticed a binder that looked rough and weighted a ton. Sifting through it I didn’t see much reason to keep it but to be safe I decided I’d go talk with my neighbor who owns a pawn shop and knows more about coins than I do. Come to find out I have 2 VERY limited coins that were nearly perfect for their time. Something about them predating US currency. Total value of all the coins is anywhere between $50k-100k. Selling them takes a life time though.