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 got one as a hand-me-down from my father’s office when they replaced all their chairs. It’s pretty well worn and the upholstery is rather frumpy these days, but the bones of the thing are still good.
I remember talking about desk chairs with a friend group and on a lark I thought I’d read off the model on it to demonstrate how unassuming this no-name chair I thought I had was. So I actually said something akin to, “Yeah, it’s just some chair from some company called Herman Miller, whoever that is,” and everyone was aghast. They had to explain to me what Herman Miller actually is, and I was very embarrassed for having accidentally humble bragged about it.