IMO, 10,000 a month is not so good there in Guangzhou, 18,000 a month alright.

I agree with what you have said. I think too many foreigners come to China intent on competing with each other and 'live to work', rather than 'work to live'. Just what Chinese bosses love, because they can make more money by employing them. And, of course, the Chinese government will rake in income tax. Best to be under the tax threshhold so that the Chinese government does not get a penny of your salary as tax payments!
I think it would be extremely useful if those who quote salaries also gave us detailed information as to the cost of living where they are located, and the cost of renting or buying an apartment there. I've heard that living in Guangdong is very expensive, compared to many other less well-developed areas of China. I've also heard that it's got loads of foreigners, which would necessarily drive down foreign teachers' salaries.