Depending on how long you have the cab, where you go and the number of people, I would count on $35-$50 total plus tip for two people. One you get past 2 people the cost per person goes down. If you want an island tour, beach time, etc, you can negotiate but rates are pretty much set. There is a taxi dispatcher at the ferry terminal and they will have up to date information.

Personally, I would go with a cab and not do a car rental.