Info on Publicos: (some not applicable to you, but worth reading)
PUBLICOS
Publicos are the cars with red boxes on the top that say Puerto Plata-Sosua (Puerto Plata is abreviated). The usual price is RD$15/person to Sosua, but they may increase that after 6:30 pm. I know the publicos that go toward Cabarete increase their price after 6:30 pm.
To get a publico to Sosua, you will need to walk out to the highway, cross the road, and wait for a publico to come along. Most likely he will toot when he sees you standing there, but otherwise wave him down. The normal load in a publico is 2 in front with driver and 4 in back, which is RD$90 for the car. Assuming there are just two of you, for example, you shouldn't have much trouble getting to Sosua. If you have a big group, you may have to split up because the may already have some passengers picked up in Puerto Plata.
Keep in mind that the driver has 6 seats he wants to fill, so you can have his whole car for RD$90, the back seat for RD$60, and front seat for RD$30. When my wife goes to Puerto Plata, she always pays RD$30 so she can have the front seat to herself (she has a knee problem) and is not squeezed in by another passenger. If there are two of you, you could pay RD$60 and have the back seat to yourselves. If the car comes and there is one in front and one in back, tell the driver you will pay for the back seat and he will quickly move the person in back to the front.
If you know where you want to go when you get to Sosua, ask the driver how much it will cost to go directly to that location. Otherwise you will be dropped off on the highway and have a walk of a block or two into town.
The publico starting point in Sosua for cars to Puerto Plata is in front of the fereterria next to the Texaco gas station. To keep cost down, it would be worth going there and asking driver how much it would cost to go to your hotel in Playa Dorada. Whatever it is, it will be cheaper than taking a taxi.