EZgamer85
CAGiversary!
I was surfing Gamefly and noticed that this was only $24.99 used (only for 360). This is the cheapest I have seen it. I know you can get a new copy for about $30, but I have had good success with Gamefly used games (over 60 purchased).
http://www.gamefly.com/Buy-Warhammer-40,000-Space-Marine/144015/
http://www.gamefly.com/Buy-Warhammer-40,000-Space-Marine/144015/