Microsoft store is now stocking unlocked Lumia 635. The unlocked device is priced $179 online and though not mentioned should support both AT&T and T-Mobile LTE bands. Though, it is available on cheaper prepaid options from both AT&T and T-Mobile, but if unlocked is what you want, this is the deal.
Coming to prepaid AT&T Lumia 635, one can grab the device from retailer Frys for just $79.99, which amounts to $20 discount on the official price.
Check our video review and in-depth coverage of Lumia 630, which should give you a fair idea about all things Lumia 635.