The short version: how much does it cost to purchase ChoiceSeats on flt. 718, non-stop PHL to FCO?

From usairways.com: "We sell ChoiceSeats by flight and the cost varies based on flight length, destination and time of day" which is not terribly helpful.


The longer version of the question: my son has been given a trip to Rome by a medical charity. A travel agent - gasp! - is booking it all for us. On the outbound flight she could not get a window seat and the seat next to it so she booked us into the center section. It's a daylight arrival, so my son really wants to see all he can from the air. I checked the seating chart for the flight, and see that there are window/aisle pairs in the ChoiceSeat section, so I'm wondering how much it would cost for me to just buy the window seat for him once I get the res # from the agent.

p.s. I don't have status with US