I don't know, how your are doing calculation.
In my KHYBER, at current LPG price with HASCOL is 64.72 Per Litre, rite now at the filling of 3000, its easily giving me 400 plus KM with AC. and if we calculate 3000 / 400 = 7.5 Per KM
Whereas in Petrol at current price of 104, it hardly giving me 9 km (with AC) so its cost 104 / 9 = 11.55.
on Average my cars runs 80 km per day x 26 days = 2,080 km per month i.e. saving of Rs 8,424 per month, for me its very big.
Previously i was running on CNG, but long line or suddenly stop of CNG, on petrol (with out engine tunning) it was expensive for me, that's why i switch over to LPG.