How much are flights from Singapore - Koh Samui?

That's a route that is difficult to fly cheaply. Bangkok Airways owns the airport in Koh Samui and pretty much has a monopoly on the flights in and out of there, only Bangkok Airways and Thai Airways fly to Samui from Bangkok. Try Air Asia or Tiger Airways to Phuket from Singapore and Bangkok Airways from Phuket to Koh Samui. From Samui to Bangkok only Thai and Bangkok Airways offer flights. Alternatively, you can search for flights and buses to Surat Thani which is on the Thai mainland near Samui, you can then take ground transportation and a boat to Samui from there. Firefly Airlines flies to Samui from Penang if you can find a cheap flight to Penang from Singapore. Sorry to confuse you, and good luck.