This weekend I stumbled onto MEX-IAH for $271 on AM or UA. (Believe Interjet had fares at this price as well.) Not sure who started the sale, but I'd guess that it's in response to Spirit, which begins flying Mexico City (TLC Toluca) to Houston in May.
Now that the sale is over, UA is back to a ridiculous $669, which AM matches on the non-stop flight. If you're willing to connect in MTY however, AM will sell for the route for ~$385.