I think that you will have to pay 400 THB / car. Taxi or minibus. That doesn' t matter.
Taxi will charge you 400 THB and in the minibus, you will pay per seat.
If you don' t have too much baggage, take a taxi. You will arrive more quickly at your hotel.
have fun
p.s. I will arrive in Samui on 28th april and stay till 24th may in
LOS jiippppiiieeeee