If you REALLY want to go crazy try looking at prices of genuine Nakamichi tapes on eBay.
I have several hundred sealed blank cassettes left over from one of the last stores I owned. A lot of various brands.
I was on E-Bay just looking to see what some of them were selling for. I have a couple of cases of the Teac CRC-90 tapes that look like tiny reel to reel tapes. I auctioned off 2 tapes that were still sealed, but had small tears/rips in the wrappers. They sold for $130! People are crazy for some of these...