United has round-trip flights from Austin (AUS) to St. Louis or Kansas City (STL/MCI) for $114 with a connection. $57 each-way. Flights to St. Louis from January to May. Flights from Kansas City from January to August. Weekends available, mostly in January and February. Origin/Destination reversible.
Note: “Basic Economy” fares do not include a free carry-on bag. Only a personal item (like a small-backpack or laptop bag) is free. Checked bags cost +$25. Seats are randomly assigned prior to boarding. If you have the United MileagePlus credit-card, use the “free checked bag” perk.
Note: The personal item size on United flights is smaller (9-in/10-in/17-in) than Frontier or Spirit (8-in/14-in/18-in).