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 $225. I could possibly even get towards the higher end of that number since I still have everything in the original box in good condition. It’s not worth a ton compared to some other items people may own, but I think most people would not expect nearly that amount.
Oh, I always win at Book Off in Japan. If you don’t know what that is, give it a search. It’s an interesting place.
I’ve bought several expensive camera lenses for 8-20$. Since they have no electronic components, they work fine. I use them to document work I do for various people or myself as a marketing too for my business. Worth every last one of those 8 dollars! Some are worth quite a bit of money.
There’s a vacuum tube on my desk worth a bit. I found it for 3$ in a junk bin. Turned out it worked, so I built a weird, cursed amplifier out of it as a joke, using some old Soviet scrap and mystery Chinese ICs. Probably not worth anything anymore! – but hey, it’s a tube amp that works entirely at 5V! So weird!
I have a beautiful set of unused old ink stones from a famous manufacturer in China. I paid around 10$ for it. These are actually quite expensive and worth hundreds of dollars. Certainly less than a thousand though.
I also have a singing bowl, made of cast bronze. I don’t know much about it, except it’s old enough to predate modern machining (it was clearly sand-cast). It’s probably also cursed – someone sold it to me by accident for a few dollars when I asked for something else. Then I didn’t notice until I got home. It’s probably worth some money to the right person, but few people value such old things in my country and I don’t want to sell it to an overseas buyer.
Oh and I have one of the original victory fliers from when the Japanese defeated the Russians in 1904. In perfect condition. I have no idea what it’s worth, but certainly much more than I paid for it, haha. I should probably find a museum for it one day.