A friend of mine has a TEFL but no degree. However, his English is very good and I believe he earns just over 40K. He lives quite comfortably on that and he's also aware that this is a short term career.

Depending on your lifestyle 40K is plenty. I've lived on that with no problems and still had money spare at the end of the month. I don't drink, smoke or use dirty women though. For me, the money isn't important (paying the bills etc are) but the free time I have here in comparision to what I had in the UK is.

If your coming because the booze and women appeal to you, then a better paid job is advisable.