Santiago is no problem. We have had people before in our apartments flying at Santiago. We hired a taxi for them who brought them here: US$ 80,- per transfer (1 - 4 persons). This is the easiest way. You can hire a car, but I think you will have to bring it back in Puerto Plata, so then you need transport from there to Cabarete. The road is not difficult, you just have to get used to the driving style here
The cheapest way is to get the bus and go with Caribe Tours. I believe you can get on it at the airport or close and than you go until Sosua. From there you take a taxi or ask your hotel to have a taxi there waithing for you. Probably about US$ 10,-.
If you need more info about Cabarete or the DR, you can always send me an e-mail, no problem. My e-mail adres is email@example.com