American Airlines and Delta have round-trip flights from Dallas (DFW) to Mexico City (MEX) for $283-$295, NONSTOP. Flights April to June. Departures on Monday, Tuesday, Wednesday, Saturday, and Sunday. Very few extended weekends available.
A Carry-on bag and also a personal item is allowed for free on Delta. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.