Cost is 350 processing fee - 30 for postal stamp - 100 for red crescent donation.
Postal stamp can be had from Pakistan Post Offices but the Deputy Commissioner's Office in G-11/4 will take the money on their behalf and take care of postal stamp for you.
Need original passport, visa, CNIC, valid Islamabad drivers license, 2 photographs of 1x1 size and completed application form.
Make sure to take 1 copy of passport, visa, CNIC, and valid Islamabad drivers license.
Hours are between 8AM to 1PM for collection of applications, and they return by 4PM the same day or the next day.
You can request urgent processing if you provide proof that you have a sooner flight to catch.
Must appear in person ONCE at least - best at the time of application.
They will take a digital photograph of you, to print on the computerized IDP they will issue.
Exact location of the office where you apply is below:
1- As you enter from the main gate, take slight left and enter the building through the door on your left.
2- Walk up one flight of stairs to go to the first floor.
3- Turn left again and I think the first office to your right has a couple of desks and people sitting there working on IDP applications.
4- Ask nicely and you shall receive.
As for whether you can get it without a valid visa - you can ask, but they want to make sure that a visa is applied for at least.
Hope this helps.
Great thread OP.