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 have a game boy game that was worth over $30 (which was fairly high for a game boy game at the time). AVGN made a video on the game and it shot up in price more than tenfold. Though it may have cooled down since I last checked.
That’s cool. I wish I had kept some of my old Gameboy games, either to play or to sell. A lot of the games I had have jumped in price in recent years.
Especially Pokemon games! And especially if you kept any of the cardboard!