Above is timetable of ferries come to Ko Lan and back to Bali Hai pier.
One way is 30 THB/pax. It takes around 40 minutes to reach the island.
You can pay 180 THB for round trip: you will be dropped off at specific beach and picked up from there for return transfer.
You can book speed boat to Ko Lan as well. The way will take 20 minutes. The total is 2,000 THB (up to 8 pax).