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 discovered that a small strip of dead land at the side of my friends’ brand new house was available to buy for loose change from the original land owner (a once giant estate that had been broken up into parcels at auction) Fast forward three years or so later and the developer started stage two of the house building, another 850 houses. Guess whose tiny parcel of land was needed for utilities and access due to the terrain? We basically picked a number out of the air and the next day the cheque / check arrived. Paid off the mortgage, sold the house and paid about 90% the price of their dream home straight away and had a bit left over too. Can’t help but think that somebody somewhere lost their job/contract over that.