The evolution of a software product line requires different product management practices compared to a single product since the diverging requirements of different customers must be coordinated to preserve the common product line architecture. Allowing too much variability leads to substantial follow-up costs during the lifecycle. This paper describes the challenges of product management for an evolving software product line. A costing approach is proposed to enable more transparency on the costs of variability, support sound decisions on the appropriate amount of variability and gain control on the development process.