i want to get visit visa for china, plz tell me about expenses n how i can apply?

You may apply for a China tourist visa from the China Embassy in Islamabad or Consulate-general in Karachi, but they don't accept individual visa application for the purpose of tourism in China. You have to take part in a tour group with over 5 members by a qualified local travel agency. You don't have to pay any visa for the visa application, you just have to prepare these documents:
1. Truly completed Visa Application Form affixed one recent color photo (30mm X 40mm).
2. Your Pakistani passport with a validity of more than 6 months and minimum 2 blank visa pages, and photocopies of the personal information page of your passport and recent Chinese visas if any.
3. If it is your first visit to China, you should provide your local police character/clearance certificate.
4. Your bank statement and round-trip tickets

The duration of a single entry China tourist visa is less than 30 days usually.