Crackdown isn't the only thing on sale. I just got back from CompUSA, and in addition to Crackdown, I got Gears of War for $37.49. I also had them scan NFS: Most Wanted ($22.49) and Fight Night Round 3 Platinum Hits ($22.49).
The games scanned at regular price until the cashier totalled the transaction. The original price was $34.99 for Crackdown and $49.99 for GoW, and the screen showed a discount of $17.50 and $12.50 when the transaction was totalled. YMMV on GoW and the other two games until somebody else can confirm it, of course.
Edit: GoW is still $49.99 on the website, so it's a definite YMMV on that.