in a production order there is no use of the average cost of inventory items used to produce a finished good. only the quantity is required.
other expenses not involving inventory items can be added by selecting the checkbox Add non-inventory cost into production
@Skn, the Inventory Items tab will list the current average cost of inventory items manufactured under production orders or purchased directly, without distinction. If you want to know the cost history of items manufactured under production orders, these are accessible by drilling down on the item in the Inventory on hand account from the Summary page.