My pedantry itch is kicking in and requires me to inform you all that the actual keyword you are looking for is FMV (fair market value) of common stock.

Investors set the price for preferred stock in various "rounds," so a company kinda knows what its preferred shares are worth.

But common stock usually isn't traded in any kind of market (until liquidity), so there's no obvious way to know what its FMV is.

A 409a valuation is a "professional" assessment of the value (price) of a share of common stock, which a company's board can safely use to determine the FMV of its common stock.

Anyway GGP is right that the amount subject to tax at exercise will be the difference between your option strike price (which was set based on FMV on the date your grant was issued) and the FMV of common stock on the date of exercise.

The FMV of common stock is almost always less than the price of preferred stock, though the two prices do converge as a company grows and gets closer to liquidity.