Hello Mike,
Adagio Inventory does not store unit costs for items (except in field like "Most recent cost" that are displayed. It stores the total quantity and total costs for each inventory item.
If you make your stocking units appropriate, (for example "feet" instead of "100 Board Feet"), all the costing will work properly on the assembly, UNLESS you need to add a fractional penny amount to each item assembled. (For example, the assembly of 30,000 items where there is an incremental need for .0023 cents to be added to the value of the master item on assembly.)
If you need the fractional "Additional costs" on the "Assemble Items" screen, then this functionality is provided by the
Adagio BillofMaterials add-on. It also provides auto-assembly and some other features related to BOM processing.