A programme on the BBC World Service backed up by figures from the International Labour Organisation (part of the UN) has attempted to find what the average global wage is.

By inserting the country and your wage (in local currency) into the table it can tell you where you rank against the global average wage and the average wage for the particular country that you are working in.

http://www.bbc.co.uk/news/magazine-17543356

The figures produced use Purchasing Power Parity (PPP) US dollars as there basis to try and account for differences in the costs of living around the world.

The research itself admits that there are several areas where the data is incomplete. Perhaps most notable is that it doesn't take into account the self employed or other sources of personal income. Clearly in many developing countries this means that a very large amount of income has been left untouched by this study.

However, given that most teachers will be drawing a salary from a school, university or private company it is still a useful tool for comparison of a foreign teacher's wage against that of other salary based workers in a country. Certainly worth considering when comparing job offers across the world.