You hit the nail on the head with Delta/Song. They have not "fixed" the Song into Delta merge completely yet, so the inventory in the marketplace is only the very highly priced seats. I think you'll see the $517 R/T dropping to the $250 to $275 range once they fix the mechanics of the schedule and release the other fare classes (L, U, and T).
JB isn't competing much these days either, so the best I've seen for them is $292. I'd bet they'll come down a bit, but not that much.
SWA hasn't released the schedule beyond August 3rd, but you can probably get $59 or $79 one way fares out of PVD once they do.
Allegiant Air out of Worcester has $70 each way, but doesn't fly certain days and flies in and out of Sanford.
Good Luck!!