An alternative if you're going to Hua Hin is to take the shuttle van (~9 or 10 seater) between the Grand Hyatt Erawan Bangkok and the Hyatt Regency Hua Hin. There is a counter inside the Grand Hyatt - IIRC they charge 650 baht each way. You can take airport bus A1 to the Grand Hyatt.

Or these guys charge 590 baht to/from Bangkok or 690 baht to/from the airport at fixed times: 8.30am & 2.30pm
http://www.travel-huahin.com/transfer.htm