United has round-trip flights from San Antonio (SAT) to Denver (DEN) for $131-$141, NONSTOP. Flights February to May. No Weekends. Departures on Monday, Tuesday, Wednesday, and Thursday. Origin/Destination reversible.
Southwest has competitive fares if you add the “SAVENOW” coupon-code. Keep in mind that while the coupon code at Southwest says 50% off, the base fares are about 50% higher than these. So once you do the math the Southwest fares are more expensive. You do get better bag allowance however.