The fast growth of stored data volume raises the importance of proper data storage management, for both technological and economic reasons. There is need for measures that would allow to evaluate storage system efficiency in all its aspects, not limited to capacity utilization, and including the performance impact of data compression. In this paper, more than ten technical efficiency measures for data compression and storage are discussed. The main contribution is an economic efficiency measure, which combines multiple relevant performance factors using the related costs or profits as a weight of their respective importance. The proposed efficiency assessment procedure can be applied to any kind of storage systems. It is also supported with software to gather and manage the required input data, calculate the measures, and present the results in user-customized form.