These destinations are ideal for low cost travel.
Daily budget for food-housing-inner travel will vary between $10-$30, and if you are really good, you can even lower your expenses. Bangkok and the large cities in Vietnam were the most expensive, according to my experience, because the cost for a hostel room is higher there.
You also need to get visas for Vietnam (I think $60), Laos (about $30) and Cambodia (??).