I'd also add that at Southwest it's irrelevant what the CPM is, because it's always the same. So if you've got enough Southwest points for a flight you'd otherwise pay with your own money, you might as well use them (unless there's some credit card bonus you're chasing that makes paying in that case better, perhaps), because they'll never be worth more (absent tiny little differences with taxes that may not be included on points redemptions).

Some of the other LCCs/ULCCs where both earning and redemption of points is at a simple fixed ratio to cash cost may be the same (in CPM never varying), I dunno, I don't belong to any of the others.