I'm not sure who are the middle class in China... For a white collar with 4-5 years working experience in Beijing, I would say maybe 8000 RMB per month after tax and insurance. In terms of call-center, I remember in 2008, the salary starts from 3000 RMB. I guess it would be 4000 - 5000 now.
I'm not 100% sure though. Anyone else?