1. A method of charging for printing at a management server connected to a plurality of image forming apparatus, the method comprising: setting a basic charge in the image forming apparatuses; receiving information at the management server about a recording agent installed in the image forming apparatus at predetermined intervals; determining whether or not the recording agent of the image forming apparatuses is replaced, based on the received information about the recording agent; counting a number of times the recording agent of the image forming apparatuses is replaced in a charging period, based on the determination as to whether or not the recording agent is replaced; and calculating a printing cost for the image forming apparatuses, using the counted number of times the recording agent is replaced and the basic charge as set.