Iberia, Finnair, and British Airways have round-trip flights from Dallas (DFW) to Madrid (MAD) for $500-$504, NONSTOP. Flights January, February, March, October, and November. Departures on Monday, Tuesday, Wednesday, and Sunday. 7-day minimum stay required. No Thanksgiving dates.
Some small OTAs on Skyscanner have this for about $446, which is 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.