Vietnam.

source: I've been to almost every country in SEA at least 3x. (Brunei was once, never went to Timor-Leste).

Check the forex changes and rent prices if you don't believe me.

Harder to factor in is visa costs. Vietnam, you need to leave every 90 days. So you need to buy a $25usd visa + flights/buses + hotels for 3-5 days while you get your next visa. Thailand, you only need to leave every 6mo on the DTV.