Frontier has round-trip flights from Denver (DEN) to Austin (AUS) for $39-$48, NONSTOP. Flights February to May. Weekends available. Origin/Destination reversible.
Note: Frontier fares do not include a free carry-on bag. Only a small personal item, like a Jansport City Scout backpack is free. You can avoid paying for seat selection if you accept the randomly assigned seat prior to boarding.