Back in 1988 or so, some guy working in an "authorized Nintendo repair center" saw this yellow cartridge as nothing more than a tool, just another part of his mundane everyday routine of testing the functionality of NES systems. In 2009, it's an extremely rare collectible being sold on eBay for $1,050.
This NES Joystick Test Cartridge is, functionally, a rather unexciting item: it merely responds on-screen to controller input, allowing the user to determine whether all of a controller's buttons work. It's the sheer scarcity of these items, as well as the lure of owning NES memorabilia that was never made available to consumers, that drives the price up.
If you buy this and decide you aren't done blowing cash, why not collect more NES test carts? There are cartridges for controller ports, Power Pads, an NTF2 cartridge that tests the various microchips, and even yellow-cartridge versions of Duck Hunt and The Legend of Zelda (which happens to be available for just $999).