Mac Game Store has Borderlands 2 on sale for $10 right now (75% off). You can use the coupon code macgames5off to save another 5% off bringing the price down to just $9.50.
The game comes with a redeemable Steam key as it’s a Steamworks game. It says it’s for the Mac only, but the key will get your a copy for the PC as well because it’s a Steamworks game.