Aeromexico has round-trip flights from Dallas (DFW) to Mexico City (MEX) for $255-$268, NONSTOP. Flights August to November. Weekends available. Origin/Destination reversible.
Note: Some small OTAs on Skyscanner have this for cheaper than the airline. If you plan on booking there, beware of any site that charges a fee to cancel a ticket within the 24-hour “free” period. Use unknown booking sites at your own risk.