Add "Malaysia" to your list and fly AirAsia... that's your cheapest bet
Liam: I've checked it, the railways in Thailand and malaysia is among the best in the world, and it's pretty cheap too. A bit time consuming, but you could do a lot of stops on the way like Langkawi Island (ferry) and Cameron Highlands. I think you can reach several popular tourist destinations like Phi Phi from the route as well. And yes, AirAsia is you cheapest bet on flights, though you might have head to Tianjin.