I would imagine the Red Sox are looking for a relatively cheap #2 to finish off the rotation, or go all-in on a cost controlled #1.
That means Gallardo, Garza, Fister, Shields. If Ben wants to overpay, it will someone from the Sale/Harvey/Kluber/Holland/Darvish group.
The high end group is looking too costly, consisting mostly of guys like Cueto, Zimmerman, Scherzer, Hamels, Price. Those guys are costing franchise changing talent, and give no control -- that isn't happening.