HKthe, I'm not in Hong Kong yet, but my understanding was that around $20k / month was more the going rate at a tutorial centre. However, to be fair some people I know are on this salary but their accommodation is automatically deducted from it, so their take-home pay is around $13k / month.
If you already have a job then surely you can compare the salaries and work out if it's fair? Or is your current job not in TEFL? The other key thing I guess is the notice period - if it's only a month then you could take it but keep applying elsewhere.