How to calculate period cost

I felt kind of frustrated  to understand the concept. 

Can someone help explain the calculation for the period cost , especially what it means here. 

https://experienceleague.adobe.com/docs/marketo/using/product-docs/core-marketo-concepts/programs/working-with-programs/understanding-period-costs.html?lang=en

In summary - months with no defined period costs will roll “backwards” to the last one that was defined. If there is no prior period cost, the months will be rolled “forward” to the next one that has been defined. If a period cost has not been defined for any months, reporting in RCE will not be available for the program.