It's not unreasonable to expect an average 8% return on your money if you put it in the stock market and hold it there for the next 50 years, particularly if you just track indexes.
So assuming an 8% return, and no inflation, how much would you need to live on 72k a year? 900k. Oh, wait, you're going to pay capital gains tax. At current rates (these will change) you need around 85000. That means 1.062 million.
Inflation exists. So, let's say that averages 5% a year. No problem, you make a percentage return too. You're just going to have to live on a real value of 7.6% return on investment, which moves your starting bankroll up to around 1.12 million.
You're going to want more than that, though; medical expenses, buying a house or car, college for children etc. will clean you out.
But let's say none of that applies to you. If you start working at 18 making 100k a year and using 72k of it to live on each year, after taxes you're in debt already.
So, cut back on the living expenses and say you live in a small efficiency and use about 2k a month. You can conceivably save 40-50k a year this way. It'll only take you 23 years or so to make the amount you need if everything goes perfectly, you never have a girlfriend or go out to bars and you're conscientious about saving.
Now, this time can be lessened if you have a matching 401k or something, but the return will be lower as well and there are liquidity issues there, so I didn't have that included as an option.
So do the math yourself on cost benefit. Use average salaries as a guide and recognize that money is only a small part of the reason people go to college.