To get to Cabarete from Santiago
1) take a taxi. $80, but i'm sure if you ask around at the airport (look for surf bags) you'll find someone to share with.
2) take the bus. $10 but you have to wait until 6Am when the bus starts
3) take a caro publico. $5, but unless you know spanish, know your way around and have a keen sense of adventure, you'll get lost for sure!
good luck and have fun!