Like Penny said.. you can negotiate a taxi price. My advice... there wil be a hoard of taxi/tour guys waiting for you. Go through the first group (might even be inside the terminal entry) and out to where the independent taxi/tour guys are. They are not allowed close to the ship therefore they lose a lot of customers to the main taxi companies. IF you can find some friends on the ship that want to join you you can do better for a group rate. They charge by the van full so the more people the cheaper per person rate you will get.
We met the best group of "kids" this way. They were all in their early twenties and we had met on the ship that morning at breakfast. We happened to get off the ship at the same time and we decided to join up and we got a tour van for the day for about $20 a person. it was great.
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, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013