From today on BFads.net
GameStop has Rock Band: LEGO (Xbox 360) for $49.99 - 50% instant savings at checkout = $24.99. Use coupon code SAVER for Free Value Shipping. Express yourself with a personalized LEGO Rock Den that can be accessorized and decorated to perfectly suit your rock style. GAMESTOP
Note: The promotion is officially "Buy 1 Xbox 360 Game, Get LEGO Rock Band for 50% off," but it's currently working as 50% off LEGO Rock Band straight up without any additional purchase.
Just tried it myself and the discount shows when you put it in your cart. Wish I had waited one more week for this, oh well.