[please correct me if i'm wrong at any step
so you said your monthly commute will be around 600-700 kms which means around 25km \ day.
lets take 30 km \ day.
petrol cost - 70\ ltr
diesel cost - 45\ ltr
petrol's average - 12kmpl
diesel's average - 16kmpl
yearly travel - 8400 , lets say 10000kms
petrol's expense per year - 60000
diesel's expense per year- 30000
difference = 30000
ok , brio s (o) price - 5,00,000 ex-showroom
ritz vxi petrol price -4,50,000 ex-showroom
ritz vdi diesel price - 5,50,000 ex-showroom
so lets compare if you wanted to purchase ritz only ,
you will pay additional 1,00,000 ex showroom , say 1,30,000 ( on road price ) and loan for 5 years , so total will come around 1,60,000 right.
( just imagine , you will have to pay 6-7% interest for that additional money and if you buy the petrol and invest the additional money for 5 years, how much you wil get say 5-6% , so total advantage if ou but the petrol will be 12% * 1,20,000 for 5 years)
and the price difference between petrol and diesel is just 30000 per year.
so you will cover your money in 5 years if you go for a diesel. nah.
no go. don't buy diesel. instead buy the petrol.
don't think what you will save in a day or a month but see how much you will save on the long run.
you will get a higher model of petrol with extra features.
diesel maintenance is very high.
normal petrol's servicing - 1500
normal diesel's servicing - around 5000-6000.
and if you don;t maintain it properly , you will face many problems after 3rd or 4th year.
so i suggest to stick with petrol. if you have any doubts please ask.