In-store price is $19.99 Best Buy's @Gamer magazine contains a coupon for $15 off Bulletstorm for Xbox 360 or PS3. If you are not a Gamer Zone Rewards member and get the magazine as part of your membership, you must purchase the magazine for $5 (unless you can find someone to give you their coupon).
Note that the magazine has some other great coupons as well.
It should be noted that the online PS3 price is showing at $12.99 but I could not get them to honor both the online price and the coupon together to get it for free (they would only do one or the other). I could also not get them to price match another store and use the coupon, so $4.99 is the lowest you will be able to get it unless you can find an overly helpful manager.