The listing looks like a weird bug in AT&T's website. If you click on the listing for LG's G3 Vigor, you'll see a link to the Nexus 6 in the right hand column under the "People also viewed" heading.
The photo doesn't match up with the listing, however, as it appears to be showing an LG phone. When you click the link, AT&T redirects you to the landing page for its Motorola phones rather than a product page for the Nexus 6.
If AT&T does in fact set its real Nexus 6 product page live later today, it'll be the first time the carrier has offered a Nexus phone since the Nexus S launched in 2011.
The Nexus 6 would also be considered extremely cheap compared to other flagship phones, especially in comparison to other large phablet devices. The iPhone 6 Plus, for example, costs $299 on a two-year contract. Samsung's Galaxy Note phones typically cost around the same.
The Nexus 6 is expected to come with a giant, six-inch screen with a resolution of 2560 x 1440, a 13-megapixel camera, and Android L at launch. We're likely to hear more about the phone later today.