I was looking for one of them a couple of month ago and remember the prices on Ebay. Boxed they tend to go in the $45 to $55 range.
Although you must know that the Game Brain has a bad design and it is known to kill some Atari 2600 carts. There is a fix, but requires to re-route some of the Game Brain lines/traces meaning, alot of soldering and desoldering involved.
Actually, I've never heard about the Game Brain killing games until now, I'm glad I read this. But to be honest, I don't really use the one I have too often, the Atari 2600 is built like a tank, cartridge wear-and-tear won't do much damage to it.
Yeah, here's a good read about the issue: http://atariage.com/forums/topic/164...eo-game-brain/