A 1990 Nintendo World Championship NES cartridge is up on eBay with a "Buy-It-Now" price of $12,000. There were only 116 ever made, and this is one of 90 grey ones given to semi-finalists. The other 26 are gold, like the old Zelda cartridges, and were given to winners and runners-up of a Nintendo Power contest (one recently sold for $20,000+). Check out the Wikipedia page for more details on the competition. The cartridge has versions of Super Mario Bros., Rad Racer, and Tetris on it.
See it on eBay