Bulletstorm is on sale for $11.98 at Direct2Drive. Only the US site appears to have it discounted. If you use coupon code 8DFE4F43CBEAD0DB7B8AADFC6BF06D7C you’ll save 15% off, which makes the game only $10.18. This game uses Games For Windows Live.
If the coupon code above has expired you can generate a new one from the link in the promo coupon codes page.