Frontier Airlines, an ultra low-cost 'no frills' air carrier have their Buck Fares where one-way airfares to select destination, starts at $20 plus tax/fees.
Frontier Airlines [flyfrontier.com]
Valid Monday - Thursday & Saturday.
Buy by 5/15/19.
Travel by 6/19/19
*Fare shown is one way for domestic, nonstop travel on specific dates in select markets. 3 day advance purchase required. Seats are limited. Restrictions apply.