Its not safe to drink tap water. So use bottled mineral water only. I would recommend to use Nestle.
Water in murree and giliyat area comes from open water reservoir at base of Mukhspuri. Often monkeys are sitting in walls of reservoir. You can imagine now how clean this water would be then!
Large bottle of Nestle mineral water would cost around Rs. 25 (less than $1) and is easily available. Hotels would charge much more then the actual rate.