These are the 2 cheapest options if you wish to fly and come to koh samui, however please note they do not fly direct to koh samui, they will fly into Suratthani Airport and Depart from Nakorn Si Thammart. From Suratthani Airport you will take a coach or van to the Pier, followed by either Seatran or Lomprayah to bring you to koh samui 

The average price for the ticket varies depending on how far in advance you book along with what time you want to come ticket prices range anywhere from 3,000 THB to 5,000 THB for a round trip ticket

Do expect a longer duration of journey when you choosing this option, Average time including flying from bangkok to suratthani plus the journey of the van from airport to the pier, along with the boat ride to koh samui expect to spend at least 4 to 6 hours traveling depending on the setup and schedule of the airlines and there partners. 

Please note for air asia and nok air at this time there is no offical ticketing office on koh samui or at koh samui airport. 

AirAsia Website: http://www.airasia.com/

Nok Air Website: http://www.nokair.com/

Bangkok Airways - This is a direct flight from Bangkok to Koh Samui, however its the most expensive option getting here. The average price 1 way is 4,500 THB 

Website: http://www.bangkokair.com/eng