OK, my turn to try to figure this out.

The basic premise is that if you save $300 (whether in one day or not), you are only adding a few pennies to what you can withdraw in a year day.

If you use an SWR of 4%, assuming that you don't get any return on your investment of $300, you will be able to withdraw 12 dollars extra per year because of that day's work.

But that ignores return on investment. Let's say that you get a return of 6% on your investments, and you have 20 years until retirement. At retirement, your $300 has grown to $962.14. So, now you start withdrawing an extra $38.49 dollars because of that one day's work.

But that's only for the first year. If you figure you'll be retired for 30 years, you'll be getting $1,154.57 for that one day's work.

And now, figuring that the money is still getting 6% while you are withdrawing 4% (2% effective interest rate), the investment will grow to $1742 at the end of retirement, which means that you can withdraw $70 in that last year.

This ignores inflation, taxes, all that stuff.

So, I'd say that for every day you work (or every $300 you save), with the assumptions above, it means that you can withdraw an additional $1,000-$2,000 during the course of your retirement, or between 38 and 70 dollars a year. Or between 10 cents and 19 cents a day.

So, basicly Dimwit got it right. That is for that one day, you are getting "only" less than 20 cents a day. But 20 cents a day for the rest of your life, for one day's work isn't too bad.

[Note massive editing to correct my stupid mistakes.]