Tetris (Japanese Megadrive edition)
PurchasingTetris is hardly difficult… unless it's the Japanese Megadrive version that you want. Legal issues caused it to be pulled after only a very small number were made. Our sister site Kotaku claims only 10 are now 'in the wild'.
As with Gamma Attack, recent eBay listings make an accurate valuation tricky, but the last guy to attempt to sell it chanced his arm with a Buy it Now price of$1,000,000,for a copy signed by Tetris developerAlekséi Pázhitnov back in July 2011. The seller had bought it for $11,000 so that was quite a bullish starting point, we'd say.
The cartridge was subsequently relisted in August 2011 and is now flagged as 'sold', although it's not clear whether that was for the $1,000,000 asking price. What the hell; we'll go with it.