Contact Taimoor @ 0321 3070100 . He got a very nice guest house in Isb. A cheaper alternative would be Youth Hostel in G-6
Address is
Youth hostel Islamabad
Shaheed-e-Millat Road, G-6/4,
Aabpara, Islamabad.
They charge 100 or 150 Per night. Place is Pretty Reasonable. But you need t do a membership first that would cost you another 300. But its an year Membership through you have avail their accomodation across the PK and even abroad if you upgrade it. I have not stayed with them ever but feedback from people is good