Cost-efficiency is a crucial Key Performance Area (KPA) in today's Air Traffic Management (ATM) system. Traditionally, research has mainly focused on the airport domain, with little attention being paid to the operating cost-efficiency of Area Control Centres (ACCs). This paper addresses this shortcoming and develops a framework to assess the cost-efficiency of an ACC from an operational perspective. It investigates how the resources of an ACC are managed, from the start of the planning process to the day of operation. The framework develops new metrics to assess an ACC's performance. A case study is carried out on the Maastricht Upper Area Control (MUAC) centre. Results show that, despite being one of the most advanced ACCs in Europe, the human workforce is operating at only approximately 50% of their full capabilities.