is a $75. Flights 0-500 miles is $50, 500-999 is $75, 1000-1499 is $100, etc...I'm sure DeltaMIA will clarify if I'm wrong, but BOS
is definetely $75...I've seen them sell it before.
As to the value...if you have an exit row, I'd keep it. You'd get the same amount of leg room, the F seats are marginally more comfortable on the MD
-88, and you might get a small meal in F depending on the day, which isn't really needed for a 750 mile flight. If you were in 38B, I'd pay the $75 in a heartbeat. But, as a person who's often stuck in the MD
-88 exit rows (when upgrades don't clear), it's not awful.
Also, on Delta, only Y/B/M/H/Q/S/K fares can be upgraded if you are non-Medallion. L/U/T can only be upgraded if you are a Medallion.