Electric vehicle (EV) will prevail in the near future and many EV's are expected to be parked all day or almost all day according to a survey in Japan. They can contribute as vehicle-to-grid (V2G) to primary frequency regulation in power systems. However, as lithium-ion battery or EV battery is still expensive, it is economically important to evaluate V2G frequency regulation cost. This paper proposes the evaluation method of the frequency regulation cost. The proposed method is based on the EV battery cost and longevity which is shortened by performing the frequency regulation. The battery degradation consists of calendar aging and cycle aging, and an empirical model is used in this paper. Simulation results are shown using the proposed method under the various conditions such as state of charge (SoC) of the EV battery. Those results indicate a V2G control strategy to provide the cost-competitive primary frequency regulation.