Just checked a flight ATL - PBI 4/22 - 4/24 Delta is matching Southwest. All you need is the Delta Credit Card for a free bag.

Delta.com $ 198 RT $99 one way
Southwest $ 198 RT $99 one way

Decided to check ATL -LA same dates

Southwest RT $ 362 nonstop 2 LAX
Delta.com RT $362 nonstop 2 LAX

The absolute cheapest flight I found on expedia was US Air for $302, with a plane change. Southwest flights with a stop or plane change actually cost more than non-stops at $377.