CNG can give as much as 70% better mileage than petrol. So if petrol & CNG are at 50rs/L & 50rs/kg and your car gives 10kmpl (petrol) and 17km/kg (CNG), and you drive 2,000km per month, you will have to buy 200L of Petrol or 118kg of gas.
@50 per kg/L for both, you will be spending 10,000 on petrol or 5,900 on gas
Someone whos income is 20,000 rs, spending an additional 4,000 is a HUGE thing. CNG (Y)