When I was there about 1 year and a half ago (at Bayahibe) I think the boat guys wanted to take us to Isla Saona for $100 per person. I'm not sure if they intended to just drop us off and charge another $100 to get back...So that price for the full excursion sounds a liiiiitle expensive, but if it includes a nice lunch, a guided tour, etc., etc., could be worth it.
Ken: We did a day trip to La Romana from Sosua and it was awesome! We left at midnight and drove til about 10 the next morning (with a short stop in Bonao). There were two bus-loads of us. Spent the day at the beach at Bayhibe, and departed at about 4pm. We were back in Sosua at 11 pm that night! It was a crazy, crazy trip - but so much fun!