Tickets at that price can be found in November 2016 and then again for flights in January through April of 2017. Though, with flight prices more fickle than an octogenarian with a thermostat, these prices aren't going to last all that long. However, these prices aren't appearing in calendar view on the site, and that might help them last a little longer. They only appear once exact dates are selected.
Another cheap find, highlighted by Flight Deal, has tickets as low as $259 to Mexico City from Newark, Chicago, San Francisco, Cleveland, or Washington D.C. These are a little trickier to track down, though.
To snag these, use the ITA Software Matrix Airfare Search. Select "Advanced Routing Codes" on the search bar. Enter the city you're flying out of and all Mexico City airports as the destination. For the outbound advanced routing code, enter "aa+ /f bc=O" and for the return routing code use "aa+ /f bc=O."