You have several options:
1) Bus from southern terminal: taxi to southern bus terminal (250Baht), bus to Hua Hin (every 40 mins) 128 Baht, 2.5 hours.
2) Taxi direct around 1,500 to 2,000 baht dependant on your bargaining skills.
3) Flight, around 1,200 baht (not sure on price as have never done it) 45 mins.
4) Train..... if you have the time and are on a serious budget
