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 picked up a used Bucky O Hare NES cartridge for $8 back around 2000. Had no idea what the game was, but the picture on the cartridge was ridiculous. Now it sells for $150-$200 on eBay. More like $600 if you have the box and manual.
Also have Sparkster for Sega Genesis including the box and manual which sells for around $400.
I have a handful of games like that. They were either bought new when I was a kid in the 90s or I bought them cheap in the 2000s. I might sell them when I’m 80.