If you already know in advance that you need to fly from Bangkok-Phuket on weekday flight, with Thai Airways you could take advantage with their new promotion. Every Monday, Tuesday, Wednesday and Thursday, you can enjoy a discounted flight from Bangkok to Phuket, as long as you book well in advance.
The cost of a current flight is 3,015 THB for Economy Class, but those who book before March 8 and want to travel between May 18 and July 15 or between 22nd August and 15th October, can get the flight for as little as 1,290 THB.
The discounted promotion makes THAI nearly cheaper than AirAsia. The discounted fares include surcharges and airport tax.
Make sure if you seek a discounted price that you book well in advance or you will be paying the full price.