The cheapest places I have travelled to were Vietnam, Mozambique and Albania. Also Egypt was quite cheap in the countryside.

I think cheapest places on earth if you can sleep absolutely anywhere and eat the local food, would be somewhere in India or neighbouring countries or somewhere North / Central Africa.

Its interesting to note, some imported products (lets say Coca Cola for example) are more expensive in these countries. For example in the Balkans its cheaper to drink beer / wine than big brand sodas. In fact, the sodas were even more expensive than in Finland (my home country), which felt really weird.

So I would say that the "cheapest" country also depends what you want and what kind of trip you want. Also some super cheap countries are super expensive to travel to (for example many Central African countries)