I think you guys are missing the point of the Economist article. What I learned in economics was PPP (purchasing price parity) which held that something which did not spoil and was cheaply transportable (i.e. diamonds, gold) should cost the same everywhere. This is 100% true, and an ounce of gold bullion costs the same everywhere on earth. A hamburger doesn't really fit, but should roughly cost the same everywhere and it does. The Economist article looks to show that PPP doesn't really hold for hamburgers, which we already know. It has nothing to do with cost of living in an area as most locals do not eat Big Macs.
Now things like hotel rooms are not transportable. Nor is labor. While climbing Mt Kilimanjaro in Tanzania, we had porters carry our heavy gear up the mountain for $5 a day. Try getting someone to do that in California.
If one is truly interested in cheap countries, look at cost of lodging and local food. I would guess sub-Saharan Africa and Central America are pretty cheap - if you want to live like a local! One can get a hotel room in DAR for 24,000 TSH or about $20 US.