Last year around this time I was preparing a short 17-day trip through Thailand with 3 of my friends from Dublin. In the end it could not be for health reasons that made me be 1 month off but I remember well that the best prices we could find for internal flights between Bangkok-Chiang Mai Y Bangkok-Surat Thani They were those of Air Asia.
In addition to the incredible prices, you could bill up to 20 kilos for free. Although I think it is still the best option, the luggage limit has become 15 kilos and we will have to pay for the excess. On the website of this airline based in Malaysia, you can check the different rates that you will have to pay depending on the airport of origin, although it also offers good discounts if invoices online With more than 24 hours in advance.
Although I did not enjoy those flights, my friends told me on their return that in terms of comfort and punctuality, I had nothing to envy at European low cost. The area is beautiful and deserves to travel through land transport, but if you do not have the necessary time and must fly, I definitely recommend you do it with Air Asia.