Calculating Cost of Delay helps Product Owners and business leadership prioritize Product Backlogs. That said, it is not a must-do calculations as there are other ways to prioritize the backlogs which might be more suitable. To help explain it, I transferred an example in an article I came across into a simple Excel sheet to help you experiment with different values calculating CoD. I also pasted into the Excel sheet the original article I adapted the example from.
The Excel sheet can be downloaded from https://goo.gl/Cn5LZv