At dealership they have AC gas machine which will extract the existing gas, check ac system for leakage by filling vacuum, then refill the gas. usually they charge for only what they refilled. your existing gas will be saved.
As per last year experience Gas filling charges at Suzuki Dealership was 12 Rs per Gram,
You can visit Suzuki Islamabad Motor I-9, Suzuki Azim Motor I-10, IG Motor Expressway (Near Naval Anchorage)