A private taxi is your best bet with a late arrival such as yours. You do not want to try and drive across the island at night. Its confusing enough in the daytime and total insanity at night. Current price for a taxi from the airport to Samana is around $170usd with Las Galeras being a little more.
We normally advise any of our guests that arrive late like yourself to either have a private taxi waiting at the airport or spend the night in Santo Domingo and catch the bus the next day. Your hotel should be able to organize the taxi pick-up for you.
You can also view the Transportation section of
www.samanaonline.com for other possibilities.