All of the airlines fly into ATL cheaply, so no need to book Frontier. Frontier is okay if you don't take a carry-on that needs an overhead bin, and if you don't mind paying for your checked bag. I know one person who had a bag slightly heavier than the allowed weight, which meant paying a premium for the bag.

Check the First Class availability via Kayak and see the deals on First Class to ATL. Of course we fly out of DEN, so maybe that is the difference. I have seen First Class seats for as low as $459 RT on Delta flying Sat-Sat. I will take that any time over flying on Frontier at $200 RT with bag fees, etc.