This huge price cut (via Paul Thurrott) comes on the heels of the Microsoft Store in the U.S. cutting the price of the unlocked Lumia 950 down by $150 to $399 earlier this week. However, that discount also comes with a free $99 Display Dock as well, which is something AT&T is not offering.
The carrier is also selling the Lumia 950 on its payment plans. You can pay $9.97 a month for 30 months, or $12.46 a month for 24 months.
Thanks to JohnStrk for the tip!