Understanding and modeling the variability of an existing system is a highly critical and challenging task when adopting a product line approach. Only little guidance is available for identifying the variable elements in a complex system and for choosing the appropriate level of granularity for modeling. Also, product line engineers have to find a balance between the technically feasible variability and the externally visible variability reflecting the business perspective of an organization. In this paper we describe experiences in developing and applying a value-based process for eliciting product line variability which aims at integrating the technical and business perspectives in product line engineering. We developed the process in a series of workshops carried out with our industry partner Siemens VAI, the world's leading company in plant building for the iron, steel, and aluminum industries.