The fair market value of a fee holding of land is the price obtainable for it when voluntarily offered for sale in the open market, under equable and normal market conditions, to the competition of buyers. Its fair rental value is discoverable in the same way. The fair market value of a leasehold of land is the difference between the amount of rental reserved in the lease and amount of rent obtainable for it when voluntarily offered in the open market under equable and normal market conditions.