Have used Princess' air deviation several times. It is $35 per person. I'd say that your agency is making a little extra off you.
Probably too late for you to book your own air, but I am doing the Sun Princess from Costa Rica to San Juan November 19. I arrange my own on most of my cruises and found a deal.
I am flying out of National, but back into Dulles. Yes, some coordinating is going to have to be done, but I got it for $200 less than Princess was asking.