Unlike others, IT services using packaged applications such as SAP and Oracle modules highly reuse and configure prepackaged software modules. However, the implementation and configuration situation of a packaged application project may significantly differ from another. In this paper, we propose a generic method and model for assessing the cost and benefit of IT services using packaged applications. The proposed method utilizes a questionnaire-based approach to project scoping to capture the heterogeneity. The questionnaire also helps define rules to model the relationship between effort requirements and possible scenarios of packaged application implementation. The proposed method utilizes a rule-based approach to generate cost estimations of given IT service projects. For packaged applications, template data can be pre-defined and customized by using reference process and metrics models, and solution hierarchies. Then, for a given implementation scenario, the business value is estimated by using and customizing the template data. Once the cost and benefit of an IT initiative is assessed as described, other derivative analyses such as business case analyses and pricing can be done by using the estimated costs and business benefits.