The only expensive element in a trip to SEA is the plane ticket. Once you are there, accomodation, meals, domestic transport, activities are very affordable (maybe diving is expensive, I don't know), to say the least ! The average daily expenses range from about $10 to $40.
$10 average :
In some remote areas, such as the Si Pan Don Islands in Laos, where I've spent $7-$10 per day on food, beer and lodging. That was in 2003, but it still should be ridiculously low today ! The most expensive hotels we've stayed in was $3 per night, and the lowest was $1 per night for up to 4 persons : a bungalow by the Mekong River. And the place is heavenly !
$15-$30 average :
This is the daily amount that budget travellers can hope to spend in cities, large cities of Thailand and Vietnam being the most expensive; one or two meals a day NOT taken in restaurant; accomodation, between $5 (Laos, Cambodia) and $25 (Hanoi, Ho Chi Min, Bangkok, ...), the rest on activities/bars/ tuktuks-motorbike...
$40 average :
If you want a 3 or 4-star hotel, lots of museums and paying activities, and all meals taken in restaurants.
Good prices on international transport by bus as well : for example, the 250 km stretch from Laos to Vietnam was only $5.
I have a few photos of my trip to Vietnam and Laos, including 2 from Si Pan Don, here