From everything I have seen on these boards, the cheapest is rent a car....about $60 but you have to turn it in at Galveston. Galveston Limo is very popular but I understand it is about $30 per person. If you stayiing in a hotel they usually have shuttles from airport and to pier.
Go on the Internet and check out the limo shuttle from Hoppy to Galvaston. You can reserve a roundtrip for two for about 50 to 60 bucks. My son and I are going 9/25 on Celebration and have shuttle reservations. They pick you up at the airport travel about an hour and drop you off at the pier. On return, you are picked up at the pier and returned to the airport. It worked for us in May and I expect no problems again.