This is an older article, but I notice that it has been revised in December to account for recent sales of Air Raid, Red Sea Crossing, Sears Superman, and others.
http://www.racketboy.com/retro/atari/rarest-and-most-valuable-atari-2600-games
http://www.racketboy.com/retro/atari/rarest-and-most-valuable-atari-2600-games