If, somehow, Southwest's sale with $49 flights isn't cheap enough for you, Frontier Airlines has a deal that might interest you. The budget airline is running a two-day sale with $19 one-way flights across the United States.
The offer runs from September 3 until midnight on September 4, so you've got to move quickly. The treks are available almost immediately (see the below example flight) and run through February 12, 2020. Additionally, a surprising number of routes are available. You'll find 74 with $19 one-way fares and more than 100 total routes with prices under $30.
Not only that but the airline says more than one million total seats are on sale. So, you don't have long to get a ticket, but there are ample opportunities if you're going to buy one. Cities with $19 fares include Atlanta, Denver, Las Vegas, New Orleans, San Diego, San Francisco, Tampa, and many others.