The question should be "where can I SAVE the most money teaching English?"
It doesn't really matter how much money you make, if you are spending half of it on rent and the other half on bills. I'm from America, living now in China, and I've saved significantly more money then most of my friends in America 4 years after finishing university. Cost of living VS salary
should be the first thing on your mind. The best thing you can do is try to look up average salaries in a city. For example, the average salary in Guangzhou, China is 4,200RMB/month ($8262 a year.) The average salary in America is about $29000. Does this mean all people in China are poor? No, because the average cost of living in China is also about 1/3rd of the price in America. Foreign teachers in China make at least double-triple then the average salary, so saving money is quite easy. I've been in China since 2010 so I know this market pretty well, if anyone has any other incites on Japan, Korea, or Europe I'd love to know!
(sources - http://image-store.slidesharecdn.com/e3 ... -large.png
, http://en.wikipedia.org/wiki/File:Histo ... the_US.png