The return on $100 invested in the S&P 500 depends on the time frame. Over the long term, the S&P 500 has historically provided around 7% annual returns after inflation, but year-to-year returns can fluctuate dramatically. Assuming you invest $100 in the S&P 500, make no further contributions and let the money grow for 20 years at a rate of 7% each year, $100 invested in the S&P 500 would return around $385.