Carnival charges $30 pp for transfers from the airport to the Port for both directions. This is approximately a 45 to 60 minute drive depending on traffic. There are limousine services which average $80-100 one way. This is good if you are a family or gorup of 3 or more. Renting a car probably averages $50 a day with taxes, etc. Avis is located at the Radison near the port.
The transfers really worked out great for us.