Sorry for the delayed response. I am not from Islamabad but my hubby's family lives there. My nieces and nephews attend Beacon House and International School of Islamabad. ISOI has a US style curriculum. Tuition is around $5000 per year for kindergarten. I believe the upper grades are around $13,000-14,000 per year.
I am not sure about tuition at Beaconhouse, but I know the school follows the British system. However, both are excellent schools.