Ever heard of the Microsoft Store? Now would be a good time to get acquainted, as it is running a promotion where you get The Maw for free if buy Bastion through their site. It's the same price but with a free game to boot. I hope you haven't had time to buy Bastion through your Xbox 360 or through Xbox.com yet.
As Joystiq points out, the Microsoft Store website mentions that you get The Maw for free with any Summer of Arcade game, but with a footnote that it's actually only valid for Bastion. However, this is due to a deal with Machinima who hand out the free game with every Summer of Arcade title. Hence the Machinima logo on the page.
The offer is for the U.S. Microsoft Store only and Paypal is not an option. So if you are not a U.S. resident, or if you aren't one but still use an American Xbox Live account to get cheaper games through the magic of exchange rates, this deal will probably not work for you without a valid U.S. address and matching credit card.