Nintendo 64 game in town - Turok Rage Wars Gray cartridge. It sells for about $120 but has been increasing in price very quickly.
This gray version of Turok Rage Wars gained notoriety in February 2012 when a seller on eBay listed 120 of them at $12.99 each. Turok Rage Wars Black cartridges have a flaw in the game that doesn't allow you to beat one of the levels on 2 player mode. To fix this bug Acclaim made a limited number of gray cartridges. People could mail back their black cartridge and receive the gray one as a replacement.
This was not a well known program so only a small number of people took part. In fact, so few took advantage that the game was pretty much unknown in the collecting community until this past February.
This master case of games was probably purchased from Acclaim after their bankruptcy. The exact number of copies in existence is hard to know for sure, but because it was only made for a replacement and never sold in stores this is probably the rarest of the color variants for Nintendo 64.
At current prices, this Turok Rage Wars Gray would be near the top of our list of the most expensive Nintendo 64 games.