Governments started e-government strategies to renew the public sector and eliminate existing bureaucracy and therefore reduce costs. Interoperability appears as the mean for accomplishing the interlinking of information systems, applications and ways of working not only within governments but also in their interaction with the administration, enterprises and public sector. The main source of administration costs is the traditional use of paper as the linkage element between public agencies. Integrated electronic processes between public agencies can be the solution to reduce these costs and create a more efficient public sector. This paper proposes an approach for measuring the benefit of incorporating interoperability in e-government. This approach is based on the identification and analysis of certain processes (business process modelling) and on the activity based costing method. In particular, this approach concerns the measuring of benefit of applying interoperability in e-government services.