I have driven from La Romana to Nagua on the new highway a few times, and that was a little over 3 hours. I would say you can get to Las Galeras by car in less than 5 by the route HB gave, and that's staying within shootin range of the speed limit. The roads to Sabena de la Mar are (or at least were) really bad. I would estimate it would take about 4 hours to drive there and then you would still have the ferry to Samana and from there to Las Galeras by whatever means. IMHO, it is a much riskier trip on bad roads, whereas you could drive all the way via the new highway on almost all good roads.
Coming from La Romana, the turnoff for the Samana toll road is just past the airport, and well before Santo Domingo. There are 3 toll stops on the way, and the total is something less than RD400. Almost exactly 10USD. You connect with the North Coast Highway at Nagua, and from there East to Samana and on to Las Galeras.