Is there any way to make the total cost come from the latest inventory average cost instead of PO invoices of the period selected? 

I honestly don’t even understand how there could be a use case for calculating profit margin analyses based on what was purchased in the time period instead of existing inventory. We look at margings over a year and well over half our products were purchased before that period so the Product Margin Report assumes "zero cost".

Is there a way around this? Should it not take PO value average from the period if they exist, and if not use the cost of the product as listed on the products. 

Unless there is something I am missing, this is potentially a very useful function, currently ruined by its implementation in Odoo