Go to Suzuki dealership, they will extract the existing GAS via machine and machine will measure the extracted quantity. Then they will fill the gas as per required quantity for Swift (370 ± 20 g) and finally they will charge you per gram for what they have filled (excluding the existing quantity they extracted).
Last time Suzuki Dealership in Islamabad charged 12 PKR per gram, but usually in the summer they have discount offer at 50%