Content last updated - November 2016 - please contact me if you have an update!
So you are little frustrated about the prices you've found for flights to Ko Samui? I'm not surprised, the route has very little competition on it, and until recently the ONLY airline that flew the route was Bangkok Airways - who, not coincidentally, own Ko Samui's airport.
Koh Samui Airport
Although its not obvious to the casual visitor today, Samui is very late to the Thai tourist scene. As late as 1970 the island's main crop was coconuts, and there were only limited boat services to it. Things have changed a little since then, but the Ko Samui Airport (airline code: USM) didn't open for business until 1989. Today its Thailand's fifth busiest.
The airport is located in Bo Phut area also known as Big Budha, its about 5km from Samui's most popular beach Chaweng, and close to one of the ferry routes to the northern island of Ko Phangan. Given the size of the island anyone looking for an airport hotel can really take their pick of any of the huge range of accommodations from beaches such as Chaweng, Big Buddha or Bophut. Local buses swing by the airport or you can get a shared shuttle van for door to door service.
The airport is not a 24 hour facility, most flights arrive and depart between 6am and 10pm.
International Flights to Samui
Curently Bangkok Air are the main carrier flying internationally from Koh Samui (though they do offer code shares with Air Berlin, EVA Air, Etihad, KLM, Air France). They connect to Hong Kong and Singapore. Firefly connects Kuala Lumpur direct to Koh Samui.
Flights to Samui From Elsewhere in Thailand
The vast majority (almost an hourly service) of flights to Ko Samui are from Bangkok's main airport, Suvarnabhumi. Other Thai destinations also have direct connections including Chiang Mai, Pattaya, Phuket, Krabi and Trat (for big Koh Chang) all by Bangkok Air. Thai Airways also fly the Bangkok - Samui route which may be more convenient if you are transferring from one of their long-haul services.
Alternative Ways to Get to Samui Cheaper.
It seems unlikely that direct flights to Koh Samui will be getting cheaper in the near future. If you have more time and less money alternative routes would be from Bangkok to bus or train to Chumpon and connect with a ferry service. This will cost roughly a third of what the direct flight will set you back, but will take about 10 hours longer (direct flights are around 90 minutes). Similarly you can catch a scenic and pleasant ferry (about 1 hour) to Surat Thani and then connect to Phuket or anywhere in southern Thailand by land or flying.