No this is not the reason due to which the car prices are less in Karachi as compared to Lahore. Actually the reason behind this fact is very big and that is the "Humidity in Air" in Karachi due to the Sea. This humidity rusts the cars more fastly and the body & paint of the vehicle remains not upto the mark in a few years so the rates of cars in Karachi are low.
In Lahore, the atmosphere is good for vehicles and the body & paint of the cars last long so here in Lahore the used cars are also expensive.