The guy who runs gameroomdeals.com at the thread here: http://www.cheapassgamer.com/forums/showthread.php?t=212234 occasionally has it on sale for $45. It's not up right now but I'd pay attention if that price sounds right (I myself am still waiting for a further drop). The website seems to be very reliable from what other cag's have said.
[quote name='Nobi-Wan']I'd say it's worth full price to be honest. They've got some new content coming soon, too.[/quote]
I agree. I paid full price for this game on release and haven't regretted it for a second. I doubt it's going to drop too precipitously for a good while.
I just got bought a copy from Gamefly for about $42 after a $5 coupon and 5% off membership. If you subscribe there or know anyone who does maybe you can get one that way pretty cheap.