AT&T stopped selling the phone over a year ago in April 2015, but its reappearance on AT&T's website likely means it has a few more in stock that it wants to sell off now that it officially can be updated to Windows 10 Mobile. AT&T has priced the phone, which was first announced in 2013, for $584.99 without a contract. You can also get it for $19.50 a month for 30 months via AT&T Next, or for $24.38 a month for 24 months via AT&T Next Year. The phone only comes in black.
Thanks to EJ for the tip!