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 $225. I could possibly even get towards the higher end of that number since I still have everything in the original box in good condition. It’s not worth a ton compared to some other items people may own, but I think most people would not expect nearly that amount.
In the late 90s early 2000s, it was the CD era. Everyone was throwing away or selling vinyls really cheap. I was always broken so having like 12 vinyls for the price of one CD was the way to go for me. I now have a nice collection of hundreds of albums. Lots of prog rock, jazz, etc. I remember paying 2$ for Jean Leloup’s first album. This specific one never got a repressing (and probably won’t), so it sells for around 200$ now.