Monster Party may be just another oddball Nintendo Entertainment System release to you and I, but the game is a much rarer and more valuable find in Japan. A prototype version of the game recently sold at a Yahoo! Japan auction for 483,000 yen -- just over 6,000 U.S. dollars, at the current exchange rate.

It's worth noting that Monster Party isn't your average overseas release. The Japanese version -- to be titled Parody World: Monster Party -- was never released at retail, and is a sought-after collector's item worldwide.

The few screenshots posted in the auction reveal a wealth of differences between Parody World and the game that was eventually released stateside. The killer plant boss, for instance, was meant to be a more explicit reference to Audrey II from "Little Shop of Horrors" -- the Japanese version of Monster Party adds a microphone and a karaoke machine to the boss room, which was unfurnished in the U.S. release.

Despite these differences, NES prototypes don't typically demand such high prices -- even the legendary Bio Force Ape, which was never released anywhere in the world, didn't clear $3,000. Perhaps Monster Party carries a similar mystique in Japan.