Borderlands 2 $22.49 @ Amazon (Activates on Steam)
Borderlands 2 [amazon.com]
for Mac is $22.49 on Amazon after coupon. Once you buy it, you get a key that can be activated on Steam. It's a Steam Play title, so, once you activate it, you can download the PC and Mac version!
$29.99 - $7.50 (promo code: CAGROCKS) = $22.49