Depends on the number of people in your party I think. We had a family group of 16 people, so there were 4 or 5 people per rental car. The transfers were $125 each, and I think we paid on the order of $275 for a one way rental on a mini van/ SUV. So you can do the math. The rental place is in a general store/bait and tackle shop about 2 block walk from the pier.
We also took advantage of the luggage valet service (the airline allowed you to check your bags in right there at the cruise ship and THEY took them to the airport. So we did have our luggage in the cars while we drove around.
Private message me if you want more details, I'd be happy to help out.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007