Metro Bus In Sosua
I have taken this trip many times, on both Metro & Caribe.
I prefer metro, nicer buses, very punctual, plus the Metro station is right in the heart of Santo Domingo on winston churchill street with all the malls, and restaurants.
So it will be easy to catch a taxi once you arrive,
Current price is 340 $RD pesos about $10 USD
I suggest taking the Metro as someone mentioned right in Sosua, next to Texaco gas station.
One correction to previous comment, the bus will also stop in POP (Puerto Plata) then Santiago but they both are only about 15 min.
I posted the time schedule from SOS (Sosua) they have also added two additional times 2:20 pm & 4:20 pm I think, but neither will get you to Santo Domingo in time.
I suggest the 8:20 A.M or the 10:20 A.M. as it takes around 4 hours maybe 3.5 to get to santo domingo, depends on the driver and those guys can drive, plus you do not run into rush hour, and less traffic in the mountains.
POP/STGO/SDQ 8:20 A.M.
POP/STGO/SDQ 10:20 A.M.
POP/STGO/SDQ 1:20 P.M.
POP/STGO/SDQ 3:20 P.M.
POP/STGO/SDQ 5:20 P.M.
Here is a direct link to the schedule
Metro Servicios Turisticos
SOS = Sosua on bottom right hand side.
Sosua is the closest station to the POP airport, the POP station is actually further near the center of town.
Plus Sosua is where the bus first leaves from so you will have the whole bus to select your seat.
Plus directly across the street is the public car station.
Hope that helps.