Without going into too many details, if you are planning on adding armored windows only, I would not recomend so. The glass is much thicker than standard so a lot of fitting is done. The interior is modified to fit the ballistic glass and even the hinges on the doors are changed in order to carry the extra weight without sagging.

Regarding an estimate on the cost. Without having the exacte price, you can more or less expect to pay double the basic X5.
The whole car is stripped down, and armoring is added throughout the vehicle.
One of the more expensive items is the windshield, which on some vehicles can cost in excess of $6000.