Your best bet would be contacting representative offices, and get a rough quote with a * prices may vary due to exchange rate calculation like which Shahnawaz Mercedes gives you,
Although final payment is due on/before delivery after the car arrives. Shahnawaz Mercedes does has ex-stock cars of some in demand models.
PS:
This is btwn Honda Shara-e-faisal and toyota Central
http://www.bmw-pakistan.com/me_dl/pk_en/dealer/find_us/karachi_showroom.html
Im not sure if Porsche Karachi is functional yet or not but this might be usefull too
http://www.porsche.com.pk/Service_center.htm