if u go by per Km savings, its more then half the price u have to pay for same Kms on petrol, in my spectra's case
petrol per km cost is 6.5 Rs. per Km
CNG per Km cost is 2.75 Rs. per Km
saving per km is 3.75 Rs.
if your kit costs u around 45000 then this amount will be recovered after (45000/3.75) 12000Kms
now depending on your usage in one year, u will recover this initial investment to start pure savings.
I have kept maintenance expenses out of comparison coz one opts for CNG only being cheaper fuel.
I am giving very conservative estimates to be on the lowest side