Frontier has round-trip flights from Dallas (DFW) to Denver (DEN) for $68-$95, NONSTOP. Flights August to October. Departures on Monday, Tuesday, Wednesday, Thursday, and Sunday. No Fridays or Saturdays. Very few extended weekends available. Origin/Destination reversible.
Frontier fares do not include a free carry-on bag. Only a small personal item, like a Jansport SuperBreak backpack is free. You can avoid paying for seat selection if you accept the randomly assigned seat prior to boarding.