Frontier has round-trip flights from San Antonio (SAT) to Jacksonville (JAX) for $28, NONSTOP $15 each-way. Flights from August to September, mostly on Tuesdays. Origin/Destination reversible.
Note: Frontier is currently running a promotion that will grant Elite status for the rest of 2018 if you register for the promotion [here] and complete two round-trip flights before October 15th . We think this fare is a cheap way to complete one of those round-trips. Elite status means that you no longer have to pay bag or seat selection fees.
Note: Frontier fares do not include a free carry-on bag. Only a small personal item, like a Jansport City Scout backpack is free. Checked bags cost +$35 each-way. You can avoid paying for seat selection if you accept the randomly assigned seat prior to boarding.