Every year I get this stupid price guide and every year I can't believe they keep putting it out:
http://www.videogamecollecting.com/
Here's some 2600 highlights from the 2007 edition:
Sears Adventure: $6
Atari Adventure: $15
Atari PacMan: $20
Commavid MagiCard: $45
Ultravision Karate: $45
Commavid Video Life: $45
Commavid Stronghold: $280
Answer Malagai: $25
Men-A-Vision Air Raid: $45
The most expensive 2600 cart is Color Bar Generator: $540