Malaysia
Lithuania
Latvia
Estonia

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.

I sort of go by how expensive it is to buy a litre of bottled water. In which case, Thailand is about the cheapest I've been to (more so than Malaysia, Cambodia, Vietnam, Laos, Myanmar in the region). Far cheaper than anywhere in europe. Haven't been to anywhere in africa or south america so can't compare.

Last year I spent about $300 for 11 days in Myanmar, this included guesthouses, with hot water, private guides, meals, and a few beers daily. Add to that $150 for internal flights.

Out of 90+ countries i've visited that's been the cheapest overall based on average daily spending and maintaining relative comforts.

In Europe, I found Belarus cheapest

I've backpacked around the world and when I was in Egypt and India I spent about $1000/month while doing all touristy stuff (hostels, meals, site seeing, going out, etc).

I have to second this - it's probably the cheapest country that I've been to in Europe, especially if you drink alcohol. It's often cheaper to buy vodka or beer than it is to buy water or soda. Food can be had for incredibly cheap - I remember my wife and I ate often outside of Tallinn for less than $15 for both of us. And these were 3-course meals.

I think the milk & honey days of cheap eats and drinks in Estonia are likely to end soon - whenever they start using the Euro (if they haven't already).

Highly recommended country to visit if anyone is in the neighbourhood.

Now what we need is a Budget Time Machine :D- DeLoreans are too expensive!

Problem with some people is that when they travel to exotic places:
- they feel superior to the locals as they are from the western world, and expect special service
- due to this superiority they expect the first world services
- and at the same time they want to pay what the locals pay for these first world services.

I guess, such people should just stay or visit US/Europe. Nobody is forcing them to go to Asia anyways.

Well traveled and open minded people always know - What you pay is what you get!
Nothing is free in this world - if you want to be treated special, then you gotta pay for it!

Uber sophisticated travelers know how to get spiffs without paying for them.

YMMV.

Malaysia
 

Food in Malaysia is very very tasty and very cheap. It's also relatively safe country. I've never been to Langkawi, but my friends swear it has the best beaches in the region. Try all the fruits too especially the king of all fruits - durian.

Riga was quite expensive when I visited last year !

As long as you avoid eating/drinking in the Old Town, prices in Tallinn are fairly good, but in general it's more expensive here than in Latvia or Lithuania. The Balkans are quite a bit cheaper too, on average.

However, in terms of the ratio between cost and quality of life, Estonia is hard to beat, within Europe anyway.

If anyone's ever passing through, feel free to drop me a line and I'll buy you a beer and tell you all about how great it is here :)

As pointed out by previous poster Factotum, there is no universal answer.

That said, in my experience the cheapest countries are those with "artificial" economies, i.e., price levels set by the government and unrelated to world or market prices. That is why Venezuela has gasoline for 6¢/gallon (depending on the exchange rate) and other countries subsidize bread to sell for pennies a loaf.

My nominations are Iran and Burma where a traveler can be perfectly comfortable for a few dollars a day. Taxi rides are under a dollar, restaurant meals a couple of dollars, and a decent (air-con, decent plumbing, etc.) hotel room $10-20. If you want to be cheap, you can ride the bus and eat street food for a few cents. A less-isolated but slightly more expensive country is Syria.

If you want to stay in international-class hotels, be prepared to pay commensurate prices, for which the Big Mac index is a pretty reliable indicator.

I was just speaking with someone this past weekend and feeling nostalgic for my trips to eastern Europe (especially Romania) in the mid-1990s. Hotels that were posh by Romanian standards of the time (and quite acceptable for me) were very inexpensive and food was cheap (but sometimes limited to pork). However, I especially remember the price of Romanian (and even Hungarian) wine to be quite remarkable. These days, those bargains are gone, but I am happy that I spent as much time in those countries as I did when they were travel bargains.

Iran was very cheap indeed as was Sri Lanka (I paid $6 for a double, but no hot water). D.R. Congo was cheap too (but not very safe).