You may recall the story of the guy who sold the Nintendo game Stadium Events for $41,300. Other old video games can be sold for ridiculous amounts not because of any intrinsic value, but because of their documented rarity. For example, a cartridge containing Nintendo Campus Challenge is the only existing copy.
In the early 1990s, Nintendo held competitions on college campuses and at popular Spring Break destinations. Like the World Championships, players had six minutes to play for high scores on demo versions of Super Mario Bros. 3, PinBot, and Dr. Mario.
Most copies of the game were destroyed after the competition ended, but one Nintendo employee kept his cart and sold it to Rob Walters at a garage sale in 2006.
Two sellers later, the price was $20,100! Read about more super-valuable video games at mental_floss. Link