it's called patience and keep on investing.
You only need to put 400$ every month into SP500 in order to reach retirement age with more than a million.
A million in 2060 isn't going to be enough to retire on. I'd be surprised if it was enough to buy a moderate house.
If you'd done this 40 years ago, the million dollars you'd have today would have the equivalent purchasing power of $300,000 in 1980. Inflation is a cruel master.
"The average annual total return and compound annual growth rate of the S&P 500 index, including dividends, since inception in 1926 has been approximately 9.8%, or 6% after inflation" [0]. With 6% inflation adjusted returns, that'd give you a million inflation adjusted dollars after 45 years of investing $400 a month.
The more interesting question is what will you buy for million dollars in old age.
Some common answers might be:
* healthcare, always expensive
* things for your kids, including education, housing, access to jobs
* random consumer spending including holidays or travel
* charity
Presuming you were able to actually afford saving $400/mo while living reasonably comfortably which is definitely not a given and not quite as common as expected by some.
And presuming you're not altogether unlucky in your strategy, which can happen.
You will have probably also accrued still useful assets to pass on or use. Or lost them.
While that's definitely true, maybe one should look at this another way.
$400/mo saved in a 0% interest bank account for 40 years = $192,000
$400 * 12/year invested at 10% (incl. inflation) for 40 years = $2.12 million
While $2.12mm in 2020 money is $440,000 in 1980 money (assuming 4% inflation), it's still much better than the alternative.
A couple of other points:
* Investing $400/mo earlier is relatively harder than $400/mo later in the 40-year timeline assuming compensation increased by the inflation rate but one keeps the invested amount ($400 here) constant. This is ignoring factors like salary increases later in one's career (not sure if that even generalizes for lower-income professions).
* I wonder if there's any study done tracking if there's actually a steady increase in spending consistent with the inflation rate for all individuals (with some stochasticity). Or, do people adjust the goods they buy and "outwit" inflation? For e.g., the biggest purchase one makes usually, a house/apartment, doesn't scale with inflation. Similarly, electronics actually get cheaper. So, apart from food, does spending actually track inflation?
You invest into something like the S&P 500 (maybe better: MSCI World/FTSE Developed World) precisely to outrun inflation. In the OP's example, the million would be worth that million, not 300k. Look up compound interest.
If you'd done this 40 years ago, the million dollars you'd have today would have the equivalent purchasing power of $300,000 in 1980. Inflation is a cruel master.