The cheapest DL fares out of PVD and MHT are competitive with WN, often as low as $200 RT anywhere that WN flies. But the upgradeable fares are often twice the U fare, sometimes a bit less.
When I was GM, I would never book an upgradeable fare unless I could apply a SWU to that flight. Then, at 72 hours, I would attempt to change it over to a MSU. I hardly ever played the Russian roullette game that GMs have to play today.