If you're eager to get your hands on an unlocked HTC One M8 for AT&T and you don't mind snagging a refurbished model, you can currently grab one in "gunmetal gray" for $270 on Ebay.
While the HTC One M8 isn't the newest phone on the block by any means, it's still a pretty good option for those interested in a high-end Windows Phone experience outside of the Lumia line. The device packs a 5-inch 1080p display, 32GB of storage, 2GB of RAM, HTC's UltraPixel dual camera on the back, and a front-facing 5MP shooter. All of this runs on a 2.3GHz quad-core Snapdragon 801 processor and a 2600mAh battery.
Again, the listing is for the unlocked AT&T model of the HTC One M8. If you're on Verizon, you're out of luck unfortunately. If you're interested in snagging this deal, you can grab one from Ebay for $270 at the link below.