The present value of the expected net benefits PVNB is given by:-

http://en.wikipedia.org/wiki/Fisher_equation?hc_location=ufi

For example, assume that Z_i represents the undiscounted expected net benefits at the end of year t, evaluated at constant prices, and R_t, I_t, and r_t are the real rate of interest, the expected rate of inflation, and the nominal rate of interest for year t, t = 1, ..., n, respectively. The present value of the expected net benefits PVNB is given by