Quick Help Needed, How to get to Koh Tai

Hi im in Kho San Rd and want to get a bus to to Kho Tai, i know i have to catch it from Sai Tai Mai bus station and get the a fast boat across

my question is can any one tell me how much it would cost for just the bus trip, a local travel agent quoted me 850 bhat that includes a speed boat ride,

would it be cheaper for me just to go to the bus station and get a speed boat once a reach my destination??

Thanks in Advance

Quick Help Needed, How to get to Koh Tai

where is Koh Tai?

if its down south then the price seems about right, bus + speedboat. Speedboats tend to be more expensive then the regular ferries.