Delta and Aeromexico have round-trip flights from Portland (PDX) to Mexico City (MEX) for $360-$378 with a connection. Flights August to April. Departures on Monday, Tuesday, Wednesday, Thursday, and Sunday. No Fridays or Saturdays. 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.
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.