Only one of these Nintendo cartridges exist in the world, which is why it has beaten the $17,500 gold 1990 Nintendo World Championships cartridge. The new record: $20,400. However, its price is not the strangest thing about it.
The strangest thing is that you have to modify your NES to be able to connect it, and the thing has a telephone jack built-in.
The cartridge was created for the 1991 Nintendo Campus Challenge, which travelled around 60 college campuses. It contains a minigame version of Super Mario Brothers 3, another of Pin*Bot, and a minigame version of Dr. Mario. In theory, it was destroyed after the event. However, it seems that this one had a hidden 1Up and survived the cartextimation.
Many years passed and, in 2006, it was bought by Rob Walters for $40 in a garage sale, alongside many other NES games that were later valued for a total of $50,000. The 1991 Nintendo Campus Challenge sold the last time for $14,000 in July 2009. [Thanks JJ]