Guys, i dunno about you but i'm not in the favor of CNG, although i have CNG but its not worth it.
Since your buying a car that is used and not a brand new one i dont think calculating costs over a period of 5 years is valid. you can calculate it to over 1 year or 2 years max, here's the math:
CNG cost:35000 / 24months : 1458 Rs.
CNG average(on current rate) : 3.5/km on cng=
30km daily=30*30=900*3.5= 3150 Rs.monthly or lets say 3500Rs + 1500= Rs.5000/month
Petrol=5.3*900= Rs.4700 avg=5000Rs/month
Ps: i havent' included the extra maintenance/checkup/tuneups on vehicles with CNG that goes above Rs.5000
Therefore, driving you car on petrol would be the same amount as CNG but would keep your car happy and increase your engine life plus your driving experience