I work on a development team that works closely with Product Managers. There is mutual agreement between the developers and Product Managers that there should be a business case behind every feature the development team builds.

My question is, what is an effective way to make a business case for platform-level features that have higher up front cost but will provide long term benefits?

For example, the development team would like to implement a plug-in framework. There is the higher up-front cost to implement a plug-in framework but delivering the subsequent features as plug-ins will be cheaper in the long run. This is obvious to everyone including the Product Managers. Is there a standard/simple way to express the cost-benefits? Is there a simple way to visualize it with a graph?