We just grabbed a local "tour guide" at the pier. I think we paid $20 a person (might have been $25) to be taken anywhere we wanted to go. We negotiated at the pier. No its not a short drive to see the Pitons, but if you don't want to have to be forced into seeing all the things on the tour its a good option. Even paying the "entrance fees" at some of the attractions we wanted to see it was less expensive than paying for the ship's tours.
However, we were a family of four so the person person cost is spread across more people.
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, Celebrity Silhouette 2013