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.
Just recently got what I think is an RCA RS2767IF 300W after looking it up just now. Walmart has it listed out of stock for ~$150. Now, mine isn’t anywhere near new condition considering it can’t read CDs in the 5 CD changer and has some other cosmetic issues, but for $14.99 at a thrift store, I think I got a great deal. Especially since I can plug my phone into it using aux to RCA Y adapter.