What's the deal with the imbalance of pay rates in China?

(these are estimates)

I mean why does someone in Shanghai make 10,000 yuan per month, while someone in Xian makes 5000 yuan per month. The profit margin is much higher in Shanghai, even if the cost of living is a bit higher.

Look at the estimated figures:

Shanghai:
apartment 1200
food and needed supplies 1000
transport 300
_____

so you spend about 2500 on all the basics in Shanghai, but you can bank 7500 RMB

Let's take Xian,

Xian:
apartment: provided
food and needed supplies: 700
transport: 50
______
so you spend only 750 yuan per month , but your profit margin is only about 4250 yuan, considerably less than in Shanghai.

So one can assume it is not only more modern living in a big city like Beijing or Shanghai, but also more profitable. You will also notice the figures and averages are the same if you use different cities too, such as Dalian and Nanjing, you will notice that in Dalian there is a considerably higher margin of profit than compared with Nanjing, but why, Nanjing is also a very large city and quite modern, so why is it so low compared to other cities? Why in some parts of China the cost of living is lower, but you still won't make nearly as much(banked profit) as if you were working in another city that offers a big salary even if your cost of living is up, you will still bank more if the end. Shouldn't the margin of profit be not so different?

{These figures are basic averages working for Chinese company English schools.}