We just came back from that exact cruise and after being quoted $60 pp round trip (for two of us), I figured we would be better off renting a car.
We paid about $30 for the rental per day and got free shuttle service to/from the pier. In addition having the rental car gave us the flexibility to stop for lunch and last minute souvenirs before returning back home.
I hightly recommend this option (we used Budget) and also bet you can get a better rate bybooking in advance.
RCI Sovereign of the Seas 2005
Caribbean Princess 2005
Carnival Paradise 2004