The "Fair value" of the S&P 500 in 2009 was 100, circled below. About 90% lower than it is today. The fair value is based upon an historical Price / Earnings ratio of 15.
The red line shows the value of the market level when the P/E is 15. Currently, fair value is 1360 which is 30% below the current level.
However, as we learned in 2009, ex-stock buybacks, mass outsourcing, corporate debt binging, money printing, and bailouts, true fair value is closer to 100...