how many rupees should I bring with me to Islamabad for study VISA for China?

There is no bank accountment requirements for the student type based on the rules.
For you, you need to provide your admission letter, your Visa Application for Study in China, your final diploma from your country and Health Certificate for applying. Personally speaking, if you can show as much money as you have, it will become much more easier to get your student one in the end.

If you want to know more, you are suggested to consult the embassy in Pakistan for more details.

There is no charge for the application process.