"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.

[0]: https://en.wikipedia.org/wiki/S%26P_500_Index

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.

$300,000 is a lot better than $0.