According to a poll by Columbustravel.nl (a Dutch travelling website), the following countries are the most cheapest to go on vacation to

Worldwide

1. Nepal
2. Laos
3. Bolivia
4. Cambodja
5. India
6. Vietnam
7. China
8. Indonesia
9. Myanmar
10. Thailand

One can safely conclude that South-East Asia is the cheapest in the world to go to. But let's take a look at the other continents:

Africa

1. Ethiopia
2. Marocco
3. South-Africa

Europe

1. Czech Republic
2. Slovakia
3. Bulgaria

Latin America

1. Bolivia
2. Guatemala
3. Venezuela