The elementary relationship between the specific cost of a payload and initial and final mass of a rocket and the formula by Tsiolkovsky are used to derive the relation between the cost of 1 kg of payload and specific impulse of rocket propulsion. Investigations of the function obtained for its allowedus to find the optimal values for the new cost criterion and the specific impulse. It is demonstrated that the highest possible specific impulse does not necessarily leads to the minimum payload cost.