Best deal that I know of will be starting this sunday (12/21) at Best Buy, COD:WAW for the 360 will be $34.99. That's assuming you want the 360 version. Here's the link to the weekly ad thread: http://www.cheapassgamer.com/forums/showthread.php?t=207195
Or, you can buy a discounted gift card at Goozex and get the game for about $25...longer wait though, but good if you wanted something other than the 360 version. Here's the thread about that: http://www.cheapassgamer.com/forums/showthread.php?t=208162