Enterprise Systems (ES) involve relative high investments and long-lasting implementations for its adopters. A business case (BC) is often developed in the beginning to evaluate and justify this investment. This BC explains the expected costs, benefits and risks of the ES implementation. In this paper we especially focus on the problems of the current BC development and present a design proposal based on Benefits Management (BM). Research in costestimation is matured and BM is perceived as a standard in this domain. We use an iterative design science approach to create our proposal. First, we give a structured overview of current BM methods both from practitioners as well as academia using the methodological framework of Avison and Fitzgerald [1]. Second, we derive our own improved BM method based on the work of Ward and Daniel [2]. During the development process, in five iterations, we interviewed and collaboratively worked with experts towards this method.