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.
According to pricecharting.com my video game collection is worth in the neighborhood of $20k. Now, this is kind of bullshit for a few reasons, many of which come down to “just because it says the games are worth that much doesn’t mean you could actually get that much by selling them.” And I have no intention of selling anyway…i use pricecharting to track what I have, not for the supposed value. But it’s surprising to see such a high number considering there’s not a ton in my collection in terms of super rare stuff…the highest value things I own are ~$200.