We got a minibus from Railey to Khao Lak for 450 baht each. I would expect it would be very similar from Phuket. Thier are plenty of "travel agents" around offering seats on these.
I'm afraid there is no minibus Phuket airport to Khao Lak .
This limo service shows a price of 1700 - http://phuketdir.com/palbuco/rate.htm - yikes! The cheapest "limos" are Camry's not Mercs - I've found the taxi counter can usually cut them by 30-40%. To get to the taxi counter turn right out of the exit door to the terminal and walk maybe 50m.
Plenty of Ranong and Takua Pa buses (some but not all Surathani buses) leave from Phuket town bus station and go thru Khao Lak - I reckon you are looking at 150b. Or near. But Phuket town bus station is 30km in the OPPOSITE direction. The buses actually pass near the airport on the main road - you could get a taxi to drop you at the bus stop, but picking the right bus is not all that easy - they are supposed to have destinations on the front in English script but I've found not all do.
Most Lak accommodation places have a pick up service. Two years ago I paid 1000 by memory to be picked up by Poseidon. This will be a fair bit higher now.
This is a Khao Lak taxi service - you could contact them for a price http://www.khaolaktaxi.com/
we used this man
He calls himself Boss (as he is) at Cheaper than Hotel Taxi
Local Mobile: 085 7861378
He gets lots of positive reviews on Trip advisor, the cost was 1000TB, picked us up at Phuket airport and drove us to our resort in Khao Lak, I would recommend his services.