1) Take a taxi to Calle Maimon in Santiago. On one corner you have Metro Bus, they have buses leaving every hour for Santo Domingo
2) On the other corner, you have Caribe Tours, And, yes, they have buses leaving from their station every hour, too.
3) The reason for going here is that you leave straight to Santo Domingo and from the main CAribe Tour terminal you will have to get on the bus, travel for 20 minutes across town to the Maimon St. terminal, and then wait for the people to get on and then go to Sto Dgo.
4) The trip to Santo Domingo is 1.5 hours long...
Two question:
a) What time do you arrive?
b) Where are you going in Santo Domingo
Question "a" will let me tell you what time your buses leave--assuming a decent amount of time from arrival-immigration and customs + taxi to the station. And also let us know if it is a peak hour when seats might be hard to come by.
Question "B' will let us check which bus line is perhaps a bit better suited for you, since one is closer to downtown Sto Dgo and the other is closer to some of the hotels..
HB