Aeromexico has round-trip flights from San Antonio (SAT) to Mexico City (MEX) for $227-$259, NONSTOP. Flights August, September, October, November, and January. Weekends available. Origin/Destination reversible.
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.