Product durability impacts both the environment and the economy. Companies are changing their business models to the circular economy. In this model, the ownership of the product remains with the manufacturer. With this new paradigm, determining the life of the product becomes even more important for the success of the business model. The metric defined as the Marginal Cost of Durability (MCD) determines the cost to increase or decrease the life of the system. For a system to last longer, more materials are needed to counteract the fatigue damage. While this metric has been defined and used in studies throughout the literature, there is a need for a formal method of collecting this data. This paper presents a novel method for measuring the MCD aided by Metamodel-Based Optimization. A case study is presented to demonstrate this method when applied to a wind turbine tower. The results indicate that there is an increasing linear relationship between life and cost. A wind turbine tower designed for 80 years has 34% more mass and cost than a 20-year design.