I would imagine, because of the value of the currency, you'd make more money in the more developed countries. If you teach in the Middle East, the countries with oil money probably pay well too. You can make a lot of money teaching in Iraq or Afghanistan, but.....

Another thing to think about is how well you can live in that country on your salary. If you need hard currency to pay off your debts, child support, etc., you're probably better off in a rich country. But if you just want to live in that country, you may do better in a less developed country.

This is mostly conjecture (an educated guess).