Salary ranges are huge in Shanghai, from very very low to very high. So on the limited information you have given it is hard to come up with any number.
But given that it’s by the sound of it small business, I wouldn’t expect miracles.
Also, before even talking salary you want to make sure that they are able and willing to get you a proper work permit.
Coming back to the salary. What is ‘decent’?
I am sure there are many (local) chefs in Shanghai below 10k RMB a month which is not a bad salary by local standards but far from what I would personally call decent nor something a Westerner could live off.
Question is: What is your motivation to come to Shanghai:
No job back home?
Making good money / savings?