My friend told me that 40RMB per kid per class is on the low side. 50 is average.
So: 40 kids per class multiplied by 40RMB is 1600RMB per class.
and multiply that by 20 classes a week is: 32,000RMB
and multiply that by 4 weeks in a month is: 128,000RMB a month.

Those are training center stats. They don't apply in the public sector!
If you're after big bucks, you'll need to become a private training center lackey, with all the associated negatives, such as teaching in a goldfish-bowl environment, your continued employment being subject to rich overly critical parents and their little emperors who aren't qualified to assess teachers, your having to working evenings and weekends, the necessity for producing several lesson plans per week, and maybe advertising/ handing out fliers outside a foreign fast food chain restaurant at lunchtime at the weekend. Besides, if you can't attract mor students, you'll be history fast!

Public school parents pay much less for your services than parents of training center students through the school's management, so the school isn't in a position to pay you a heap of money!

Big money in China means working very hard, I reckon much too hard, as far as teaching is concerned. Become a tc lackey if your goal in life is very high pay, that's the only way to achieve that in the teaching profession.