With the introduction of Air Asia and Tiger Air and the other low cost flight providers like value jets flights in the South East Asia region cheaper than they have been in a long time. the cheapest flights to Saigon Vietnam and Phnom Penh Cambodia are from the value carriers in addition as the major carriers in the region specifically Thai Air Bangkok Air and you can find substantially cheaper flights on these main stream carriers as well.
The budget flights tend to be very early in the morning to Bangkok and Phnom Penh as they are looking to move the people to the connecting flights in Singapore and Kuala Lumpur there are also cheap flights to China as well. so you may be getting to the airport real early in the morning like before the birds are awake. Another draw back is the weight limits if you need to move anything it may very well be cheaper to sleep in and catch a flight with the main stream carriers like Bangkok Air or Thai Air.
A little info for if you are traveling in Asia backpacker areas are ussualy close to transportation hubs a good example is Bangkok's Khao Sahn rd which has access to the river which connects you to the sky train, there are also lots of regional buses will make there way to Saigon Vietnam bua vien Phnom Penh lake Boung which is pronounce like a Bong Vientiane in Laos and for the round the world backpackers the new bangkok airport for the cheapest international flights to places like Sydney Kings cross in australia and Waikiki Hawaii on Oahu and allot of other places as well.