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.
10 years ago or something I bought a collection album from an artist I liked. I just did it because I like collecting CDs and I liked his music, but it turns out that it was a limited edition and only 1000 were made, each one had a unique number on it. I’ve seen them go for a few hundred on discogs a few times. I don’t intend to sell mine though, it’s in a pretty used state because I didn’t really realize the value. I’d also rather have it than have a few hundred euro’s.