The cost of USPC for MAN will be high as I believe the US are no longer funding staff so for MAN to fund such costs and make a profit they need to ensure it's competitively priced as it adds to carriers costs which passengers will have to foot.
Have the US officially confirmed it yet (or at least publicly)?
I wouldn't see it as a "make or break" perk for a carrier to be sustainable or not. Simple fact is profit must be there without it. Big concern is once current £/$ hedging runs out from the US carriers they could be hit massively.
The consumer is almost totally driven on price of tickets (followed by brand loyalty), I know nobody who would pick USPC before departure over cheaper tickets. As for regular travelers it dons't matter where you do it as it's pretty quick.