You have a choice: you can buy dinner with your $20 bill, or you can buy a plane ticket. How hungry you are probably affects the calculus here -- but seriously, who wouldn't choose the ticket? And right now, that's a legitimate decision you can make.
Frontier Airline's current sale is offering flights starting at $19 one-way, inlcuding routes like Memphis to Atlanta; Chicago to Minneapolis; Salt Lake City to Las Vegas; and more. Other notable deals include flights from Los Angeles to Chicago for $89 one way, and Houston to to Las Vegas for $59 one way.
If you want to grab one of these seats, you'll have to book by Wednesday, March 9, 2016 and travel during the period of March, 29, 2016 - May 24, 2016. Travel is also restricted to certain days of the week, which you can check out here.