FAA.gov says Southwest has 546 737s... I don't know how accurate that is.
Let's say they get 50 737-800s.
The average flight attendant there brings home 30/hr, x 110/mo is about $3300, let's not even throw in per-diem, or open time.
3300x12 is about 40,000
40k x 50 is 2,000,000
2,000,000 x let's say 4 per airplane is 8,000,000.
That's 8 million they save by not having another FA.
My numbers are all just a guess. Not sure if the 8mil would be covered by the increased revenue of having a bigger plane.
I know 8mil really isn't that much money in the grand scheme of things either for an airline as massive as Southwest.