We paid 20 $ for a whole tour of the island.
We have been four persons and the price was really good. The driver stopped at several beaches, we were asking for.
There are taxi dispatchers close to the pier. Talk to one of them and tell him what you want and he will offer you a price and send you to one of the drivers.
We have done that on every caribbean island and never had problems and never paid more than 25 $ per person for a whole tour.