Some retro games you buy to play, others you buy to play a couple times and then put them behind a glass case.
The Gentle Physics and Science of Hazardous Materials is less a game, more a collector's item.Developed by Konami in 1990 for petroleum company Idemitsu Kosan, the title is an instructional tool for employees and never offered for retail sale. To run the game, it's necessary to use an adaptor called a "Q-ta."
Initially, bidding started at ¥1,000 (US$10.50) on Yahoo! Auctions, but skyrocketed. The closing price? A whopping ¥401,010 ($4,213). For those interested, here's a list of rare, never-for-retail-sale Famicom games. Like most things in Japan, it's in Japanese.