It seems there are too many variables to produce a one-size-fits-all answer to the OP's question. The "cheapness" of any given destination is subjective and depends heavily upon the objectives of the individual traveller. If one's desire is to stay exclusively in luxury hotels, for example, then Malaysia may well be the cheapest place - or at least it was 5 years ago when I was travelling there and getting rooms at the Ritz-Carlton in KL for $89 per night (room-service breakfast included). I don't know anywhere else on the planet where such a thing would be possible.
If, however, one is content with budget accommodation and wishes simply to travel for as long as possible as cheaply as possible, then one would almost certainly do better in neighboring Thailand, and probably better still in Cambodia. It all depends on what you're looking to buy for your money.
Then there are places that may be cheap, by Western standards, for locals, but are expensive for visitors because the latter are a captive audience. Much of Africa is actually fantastically expensive for Westerners to travel in, because there is little or no local market for the sorts of services that foreign visitors expect (e.g. high-end hotels), and because local conditions make it more expensive to provide such things (e.g. if your Western-style hotel has to generate its own electricity because the local supply is not reliable, you're going to pay for that). If you look at the "global cost of living" surveys that come out every year - I believe the Economist does one, actually - Douala, Cameroon consistently comes out in the world's top-40. Why? Certainly not because the locals are shelling out big bucks for everyday items, but rather, because those surveys are based on Western expatriates consuming the goods and services that Western expatriates consume back home, and to recreate the Western experience in Cameroon is very, very expensive. Certainly, at least, looking at the prices people pay for safari packages in various African countries, makes one's wallet long to get back to good ol' Switzerland.
Then there are still other places where meaningful comparison is not possible because the market for visitors is artificially restricted. The consummate example would be Bhutan, where the daily price to visit the country is set by the government, and that's that. In places like that, the inherent "cheapness" of the destination isn't worth considering, because it doesn't matter. So... as with anything else, YMMV.