Well, it depends upon engine (power), installation of CNG and Setting of CNG.
If engine is small i.e upto 1000 cc and CNG is factory fitted then it will be very economical e.g alto, mehran....
If Engine is Efi in this range then it will much economical e.g cultus
If CNG is installed in Heavy Bus/Truck then it will consume CNG equal to the diesel(in term of cost) and outcome will be almost zero.
So, success of CNG is inversely proportional to size of Engine(cc)/Vehicle where installation and setting of CNG is also important factor.