Look in the Ports and Hotels section of Chruise Chat.. there were discussions on Galveston from Hobby not too long ago. I think someone said they rented an enterprise car and droppped it off in Galveston for much less then the cost of the cruise shuttle. With 10 people figuring $65 a piece for the cruise shuttle, that's $650 you can easily rent 3 SUVs from enterprise or AVIS for half of that.. just make sure you do so before hand and get a confirmation number.
As far as the airfare booked through the cruise line, I have got to the point where I don't even bother asking what their price is and go straight to the airline fo rmy air travel. It always seems to cost half when you buy the tix direct.
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