Today's companies are pressured to develop platform-based product families to increase variety while keeping production costs low. Determining why a platform works, and alternatively why it does not, is an important step in the successful implementation of product families and product platforms in any industry. Internal and competitive benchmarking is essential to obtain knowledge of how successful product families are implemented, thus avoiding potential pitfalls of a poor product platform design strategy. While the two fields of product family design and benchmarking have been growing rapidly lately, we have found few tools that combine the two for product family benchmarking. To address this emerging need, we introduce the Product Family Benchmarking Method (PFBenchmark) to assess product family design alternatives (PFDAs) based on commonality/variety tradeoff and cost analysis. The proposed method utilizes the Comprehensive Metric for Commonality developed in previous work to assess the level of commonality and variety in each PFDA, as well as the corresponding manufacturing cost. The method compares not only (1) existing PFDAs but also (2) the potential cost savings and commonality/variety improvement after redesign using two plots - the Commonality/Variety Plot and the Cost Plot - enabling more effective comparisons across PFDAs. An example of benchmarking two families of valves is presented to demonstrate the proposed method.