Anyone who flies to Samui will know that it is the most expensive domestic flight in Thailand. Bangkok Airways owns the airport and wont really allow any other domestic carriers to use it (they simply set the airport fees too high).
The standard airfare (they dont have business) is a little over Bt3500 (inc taxes). There are currently 2 promotions - 1. Bt2500 (one way) running until the end of November and 2. Bt2000 (one way) lasting to the end of December.
The Bt2k fare is limited to very early morning flights there - like 6.00am (!) and reasonable return flights in the evening - like 8.00pm. The Bt2500 ticket covers most times. You must book on the web - at: