Can't help much with the airport to Galveston thing.... I always drive my own car. But the hotel- There are several hotels on the island that offer shuttle service to the ship. Holiday Inn will let you park your car (for free) in their lot and shuttle you to the hotel. However, the prices fro hotels on Galveston island are higher compared to those inland. We normally stay at something closer to the Johnson Space center area and then drive the last 25 miles in the morning.
$200 for a limo seems high. You can park a car in Eazy Cruz parking for $50 for the week if you reserve a spot on line. You may consider looking at renting a car for the week from the airport and using it both ways and parking it. OR stay at a hotel near the airport and stay overnight and then go back to the airport to catch the Cruiseline bus.. (not sure how much that is) Many of the hotels close to the airport offer shuttle to and from the airport. So that way you only have to pay for the cruiseline bus...
Lot's of things to price out. Good luck!
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