What I say about that is that you also spent at least 30,000 rupees for the CNG Kit (even if you bought the car preinstalled with CNG).
I only support a car that comes with pre-installed CNG from the factory. They are the best ones!
That cost also counts. The thing is that I have heard some very tragic stories of getting brand new cars ruined by the CNG installers and some very hard time for the owners of cars.
If I can get about 180 Kilometers in 500 rupees of Petrol, this means there is no need of CNG. The car remains covered in the warranty of the company. It also remains in factory settings for the longest possible time.
Thirdly, if I occasionally have to race with someone, I already don't have a cylinder in the car? So the power to weight ratio is optimal.
Fourth: In my family, most people with CNG and 1.3L - 1.6L cars get around 150 - 160 Kilometers per cylinder. I believe this is the average 90% of the people are getting. Remember, I am referring to MOST people. So, if my average on petrol matches with the average 'most' people get on CNG, I think its a very good deal.. with all the peace of mind and all the fuss-less driving. I put about 1200 rupees of Petrol in Rawalpindi and I don't have to put petrol again in Peshawar when I am coming back. CNG also needs to be refilled again and again and petrol needs filling less often.
These are all the small advantages I can think of that makes a car nice to drive on Petrol as compared to CNG.