You are maybe considering using the plane for inner connections within your journey, or if you wish to go to the neighbor countries. So be aware of the local low cost airlines.
Actually, there is this one company that’s really worth it if you are not planning on spending all of your money on some expensive flight : it is Air Asia. The prices are really cheap, you can fly for $40 up to $90 if you are doing well. And remember that the more in advance you are booking, and the cheaper you will pay.
But this specific company has plenty of connections all over Asia and I think it is one of the cheapest, if not the cheapest.
You can check the fares directly on the website and do simulations.
Be aware of the other local companies
For my Phnom Penh – Bangkok flight, I booked at Cambodia Angkor Air for the reason that this flight was cheaper than Air Asia when I bought it. But this specific company is very small and has very few connections.