From the western suburbs, it costs $4.50 to park at Metro, then roughly $8.00 (rounding up slightly) for roundtrip fare. That'll be my baseline - if it's much more expensive than Metro round trip, I'll be hesitant to use it. It sounds like a really good concept, so I hope it works out.
I imagine they will have to have more than one bus. I can't imagine a bus leaving Dulles Town Centre, going down 28 to Centreville (a horrible mess during evening rush hour), than in on 66 with a stop in Ballston en route. Does anyone know if there are more stops to be announced?