Kalar Kahar would be a 2 1/2 drive on a coaster from Lahore interchange.
Toll would not be that much, considering that the toll is Rs 90 for a motor car. So, toll for a coaster would not be more than Rs 200 (probably less).
Since you are planning to go to Kalar Kahar (there's not much to see except the lake), you may also like to check out the Khera Salt Mine. Take the exit before Kallar Kahar (LILLA Interchange) and follow the sign to Khera. Driving time from Lilla Interchange to Khera is 22 mins.
Entry ticket for the salt mine is about Rs 100 per person. This includes the train ride to take you 2 KM inside the mine. If you want to walk, then the ticket is much less. But i would not recommend it. The total trip inside the mine lasts about 2 hours. And its a must see!