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."