I make that trip often.....You should find a cheap hotel in Santo Dom for overnight stay, about 30 us for ride from airport to hotel in SD and then about 20-30 us for cheap hotel.........Around 150 to 200 pesos for taxi ride to Metro or Caribe Tours, from hotel and less than 10 us for bus ride to Puerto Plata.......
Metro will drop you off in downtown Sosua, if that is your destination and if Caribe Tours, you have a slightly longer taxi ride from bus to wherever in the area.......
Only other way is a taxi or whatever, at night........
and if you take Caribe Tours, keep an eye on your carry on and bus, when they stop for refreshments in Santiogo, because they changed buses on me one time when I went to the restroom....luckily I didn't lose anything, but I will never forget the experience.........