Brian,
The vertical drag depends on the wing area, as well as how much wing is under the mid-span and tip where the high velocities are. Generally, it is possible to lose 10% of the gross weight of the aircraft to wing drag (the typical helo loses about 4% or so0
The V22 probably loses 10% of its gross weight, so it suffers a 6% greater loss than a regular helo. I think you could use that 6% as a rule of thumb. 6% MGW means about 14% payload loss - 600 lbs or 3 pax for an S76, 6 pax for an S-61/92. Big deal, unless the revenue is balanced against an increase in transport efficiency.
Bear in mind that is just for the wing, the thruster/prop is an extra burden, pound for pound, since it adds nothing to the hover payload, it is a dead loss that must earn its keep in higher speed. That is why swing TR's and ring tails were attempted to replace the TR. If the combination of wing and thruster leaves 20% of the payload home, it had better add 20% to the transport efficiency to break even. That is perhaps 40knots of increased cruise speed (for the same fuel flow).
I stand corrected on the demise of the project, here is the text that Ian's link pointed to (for some reason the link didn't work):
Tuesday, March 27, 2007
Piasecki Readies SpeedHawk for Ground Runs
Piasecki Aircraft has moved its SpeedHawk compound-helicopter demonstrator to
Boeing's flight-test facility in Wilmington, Del. in preparation for ground runs in April. The modified
Sikorsky Aircraft UH-60 includes Piasecki-designed fixed wings and a vectored-thrust ducted propeller mounted on the end of the tail boom. Essington, Pa.-based Piasecki aims to achieve speeds in excess of 200 kt with the design. The company in the past developed the design with U.S. Navy funding. The latest work is being done under U.S. Army Fiscal 2007 funding.