I will try to provide a simplistic answer philosophically than mathematically.
If one could restrict oneself to only needs and removes every want, then I guess what a couple earn will be sufficient. The couple can spend a little money for their children, but they should not, live a minimalist lifestyle, for as long as required.
I know that to enjoy and experience a few things, we need money, but also there are a few things that don’t come with a price tag, we can embrace those things and enjoy life in a different way, and if this done correctly helps a lot in understanding the depth of life, the complete meaning of being born an as human.
Not to mention utilizing one’s full potential, both in climbing the ladder in professional life, getting promotions, bonuses, and with other things too.
So one can be philosophical which takes clarity and sacrifice, and one can be practical which takes effort and time. One can enjoy life both ways.
It depends on many factors
Whether the family lives in a metro city or a small town/village
Whether they send their kids to an IB school or a Municipality school
Whether they eat out at roadside dhaba or a 5 star and how often
Whether they travel by Business class flight or an Ordinary state transport bus and how often
Etc Etc Etc
There is inflation that is reported and then there is disproportionate increase of cost in many things over the course of the past 35 years. So despite some mathematical base that we can have, it is very subjective, because of the changes happened both a personal level and societal level.
I can live on 15,000 per month in a small town, paying for food, rent and all the bills. Someone can live on 12,000 too, someone else cannot.
It would be approx 15,000 today. (1500 in 1990 as base)
check out this calculator. It might give you some insight. It is called inflation calculator.
Not sure if this correct but my guestimate is that it should be.
A daily laborer in Kerala get a minimum of 1,000 per day. This means approx 25,000 per month. Of course this cannot be a base for other states as daily wage is very high in Kerala.
According to me, the average cost of living in Kerala(Trivandrum) for family is 30K.
Housing or Rent - (10500+1750(maintenance) 12250 Rs
School fees and Bus- (24000+10000 (yearly)) 2850 Rs
Food and Groceries - 3500Rs
Utility Bills, Cell Phone, Internet (gas 800rs, Electricity 450Rs,Cell phone recharge 130Rs,Internet-562Rs) -2000Rs
Clothing and Personal Upkeep- 1000Rs (Monthly average)
Entertainment - 2000Rs
Insurance - (Health- No insurance, Term insurance -1250rs Vehicle Insurance 6700Rs(yearly)) 1850Rs
Travel Expenses(Petrol and bus charges) 3000Rs