Best Buy sells Rage for $20, but there is a $15 off coupon in this month's @Gamer magazine (Best Buy's in-house gaming mag). The magazine sells for $5 individually and can be found in the gaming area of the store.
So, $5 for this month's magazine with the $15 off coupon inside and $20 - $15 = $5 for Rage which amounts to a $10 total (+ tax, obviously).
The only other thing needed to use the coupon is a Rewards Zone Gamers Club membership which is free, and you can sign up online.
Also, the coupons from this month's magazine are good until April 7th.