POPGirl, the latest published figure for taxis from POP to Cabarete is RD$350. However, as you saw in the dr1 news brief yesterday, rates for public transportation have been increased. It is possible that the taxi union will take this opportunity to raise their prices, also.
Regarding taxis from the airport, that is a different kettle of fish. They seem to charge as much as they can get away with, keeping in mind that the people who need transport from their have no other alternative than renting a car. To illustrate, at same time that people were reporting paying 400 pesos for a cab from the airport to Sosua, I went to the airport to pick up the mother of a friend in Samana. To be sure of having a cab, I took one from Sosua, had the driver wait for the arrival of the plan, then traveled back to Sosua. I paid a total of RD$500, or RD$250 each way.