American Airlines has round-trip flights from Dallas (DFW) to Salt Lake City or St. Louis (SLC, STL) for $177, NONSTOP. Flights May to October. Departures on Monday, Tuesday, Wednesday, Thursday, and Saturday. Extended weekends available. Origin/Destination reversible.
A Carry-on bag and also a personal item is allowed for free on American. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.