October looks like a good month for many Frontier routes (for you, maybe not for them).
They just posted a sale with some routes, like ATL-MCO or MIA, at $20 each way Tue/Wed/Sat. Also dropping to $22 OW for ATL-MCI and v.v on low dates, and some $29s
Am sure it's not just the ATL routes. You also don't have to be in Discount Den for the $20s and many of the others.
They've also had ATL-DEN at $50 RT Tue/Wed/Sat up for a few days, though some dates are gone by now. It might make sense to combine a $25 with a more expensive leg for a RT, but if you do $25 both ways the CPM is 2.08 cents. You can even do it intraday and some of the others can be done as "turns" if you want the mileage-run experience.
They ARE a no-frills, charge for everything operation a lot like Spirit, but with generally nicer planes and FAs even though the seating is cramped. Mileage is distance based and there's an elite level at 20K flown miles (can also qualify on segments) where you get a carry-on, can pick a regular seat free on booking and upgrade free to a "stretch" seat in the 24-hour check-in window if available. If you can get all that then the standard is similar to legacies except for complimentary drinks other than water (a diet Coke is $1.99 onboard). A saver trans-con award like an ATL-PDX RT would be 20K.
It's not for everyone, but you can get some amazing deals if flexible and re-live the distance-based collect-and-qualify experience that the oligopolistic AA, DL and UA have done away with (or, if you're in your early 20s and weren't around for the golden age, get a hint of what you missed)