AutoCAD Inventor :: Creating Parts With Costs That Change With Dimension Change For BOM

How do I create an Inventor part with a cost that will update according to the part dimensions changing. For example, we design products using aluminum extrusions and our cost is determined by foot length. How can I create the extrusion part to have a per foot cost that will dynamically update when I change the parts length for an assembly? It seems to me to be a fundamental need for many industries.

I will happily take any links to information that will explain this process. I cannot find anything online that clearly explains the process of creating parts with costs and variables.