Foreign tourists would be asked to pay lots of money as cab fare every where in Islamabad and especially near your hotel.
To avoid all this, ask your hotel reception to arrange a taxi for you. Also get an idea of how much to pay from reception staff.
Taxi drivers in Islamabad are so clever that they some time even rip off the locals too. So be smart and follow my tip.
Unique Suggestions: You may also ask locals about rough estimate of fare for the place you are hiring a cab for. The bottom line is that do not loose your money. Resources are limited so use them wisely.