There are very few "Hostels" per se. Usually referred to as guest houses, prices range from $2USD for dorm style room and right on up!!
But, I would think your average is going to come out around $8 USD a night for a private room and $10 if you want hot water and A/C. Many have breakfast included so you are not going to break the bank. I have lots of info on Cambodia and Vietnam if you need specifics. Go ahead and email me if you want.
anywhere from 5 bucks to 30, depending on the country and what you want' some places have free stay if you sleep on the roof in a hammock. try http://www.hostelworld.com