Liquidity premiums
An investment that can be sold quickly is less risky than an investment that cannot be sold as quickly at fair market value. For the harder-to-sell, illiquid assets, an investor will expect a higher return to compensate for the risk of being unable to quickly turn that asset into cash.
The easiest way to calculate the liquidity risk premium for an investment is to compare two similar investment options, one being liquid and the other being illiquid. For example, you could compare two corporate bonds offered by similar companies with similar coupons and maturities, but one bond is publicly traded and the other is not. The publicly traded bond would be considered liquid, while the non-traded bond would be illiquid. The illiquid bond will have a lower price and higher yield to compensate investors for its higher liquidity risk. The liquidity premium would be the difference between the yields of these two bonds.