This paper proposes a structured framework for calculating the cost of e-Government services, based on the complementary application of the IDEF0 modelling tool and the Activity-Based Costing technique. The motivation for this research effort was derived by the need to use an alternative method for the annual cost calculation of the TAXISnet e-Government services, since the relevant report published from the Greek Ministry of Finance in 2003 was based on traditional accounting approaches. The first step of the proposed methodology is the IDEF0-supported identification of the activities corresponding to the e-Government services and their initial classification as value-added or non value-added. Data collection follows, concerning cost elements and their activities using Activity-Based Costing. The paper concludes with a discussion concerning the added value of the proposed methodology framework and further research.