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 own a Planet Arlia Vegeta Funko Pop. Currently worth approximately $5000 USD.
Because my wife liked it and wanted one. We got it many, many, many years ago before it cost that much. I think it was around $100 when I bought it. I had just got a new job that was paying me way more than I was being paid before, and it was her birthday, and she had been talking about how much she wanted one for years. I figured if $100 makes her happy, why not? It wasn’t until shortly after that that the price started skyrocketing, and hasn’t really stopped since.