Tonyaod, this is indeed a very wild guess!
You are too optimistic about teacher qualification and what the schools consider a fair pay. Local and Foreigner teachers are not paid the same thing at all, you got the wrong impression coming from your own experience as a foreign teacher.
I have talked with several teachers from uni and private school and they all gave me the impression to be poorly paid, but none would put a figure. Their contract forbid them from disclosing or even discussing with other staff about remuneration. Off course most teachers will talk to each other but they can't all group together and complain about the general level of pay...that would indicate that they have disclosed their pay to someone else and would be ground for a fast dismissal for breach of contract. Charming, isn't it?
However, last year i talked with a former teacher of one of the leading private school in town. Most teachers are paid less than 2000rmb/month. If a teacher teach only 20hr/week that about 25rmb/hr, but most teachers will teach well over 30hr/week. Additionally, overtime worked is compulsory and not paid. During holidays teaching staffs are not paid.
Why to they accept to work there? Pay rate are pretty standard in the private sector and universities recruit only fully qualified teachers. Universities pay rates are a little bit higher but Chinese teacher in uni to earn fantastic money either.
So to go back to your calculus.
At most 6000yuan/semester for one teacher/class/semester (2000rmb by 3 months).
Your overhead of 30,000 are probably a bit low. I would not be surprised if they are doubled, because you have to include support staffs and premises cost. So i would put overhead between 45,000 and 60,000 per semester. I run a biz myself here, so i have a pretty good idea of overhead costs. However the overhead HAS to be divided by the number of classes, a school will not run with less than 10 classes, so that put the overhead around 4,500 to 6,000/class/semester
So this give us a total operating cost of around 10,000 to 12,000/class/semester. Pretty low you think? Well it is a number that explain why school can afford to run a class with only 3-4 student and still make a bit of money out of it. It also explain why it can be such a profitable biz, a class of 10 student brings in some serious margins!
So if you can get a visa by other ways than a school you should give a service to the teachers and pay one outside a school.