There have been campaigns from time to time to build a second airport on Samui. But nothing ever came of it.
You could fly to Surat Thani with Air Asia or Nok Air, bus to the pier and take the ferry to Samui. The airlines offer flight/bus/ferry packages which are considerably cheaper than a flight with Bangkok Airways. But they take a considerable amount of time. You’re still sitting on a bus when you could be already drinking a Leo on the beach.