You are missing the whole point. The Original Post talks about Why prices in Karachi is lower than prices of car sold in Punjab. He is not saying Why cars in Karachi are sold for less prices in Punjab. He is saying why karachi cars are sold at lower prices in Karachi and Punjab made cars are sold at higher prices in punjab.
And you are contradicting youself, first you said It has nothing to do with Demand/Supply. Later on you said registered vehicles in other cities has lower demand and hence lower prices.
Prices are ALWAYS determined by Demand / Supply situation and cost of production. If prices reduce thats only because either the demand has reduced or the supply has increased!
For me Why Karachi prices are lower than Punjab here are the reason
Supply & transportation costs are major issue. As someone pointed out, the delivery costs of vehicles transported to up country from Suzuki and Indus motors plants in Karachi makes the final product prices of new vehicles heigher in Punjab. Area of Punjab is bigger and supply/availability of dealers for total market who want to buy a car is lower than that in Karachi. Hence people in Punjab are willing to pay higher prices than those in Karachi where supply is no issue.
Karachi has good availability of busses and taxis than Punjab on a per Mile basis. In Lahore it's very difficult to get a taxi while not many buses deploying passengers to different parts of the city. Only some vans. In karachi you get lots of Busses, taxis, Rickshaws etc. Hence need for cars is lower, hence lower prices.
Threat of car theft is a lot higher in Karachi, which distract some people to buy a car of their own and they would rather prefer public transport.
Social status symbol in Punjab of owning a car is higher than that in Karachi. Hence once again higher need.
So if the need in Punjab is higher, why not supply is increased to bring down the prices there? the answer is it's not easy to distribute throughout Punjab, vehicles that are made/imported in Karachi. And transportation costs need to be recovered in the form of higher prices.