for the 2nd class overnight train, the fare is 190 baht from bangkok to chumphon.

http://www.railway.co.th/English/FareRate.asp

in the morning, u'll then need to take a ferry to koh pha ngan via koh tao (these two islands are actually the desired destinations of many people, for the full moon party and diving/snorkelling/beaches respectively), then another one to koh samui.

for samui itself, i also have a strong preference for a bangkok airways flight.

ian.