Question on cost on manufactured item

I haven't done much with manufactured items and had a question on how to handle this particular scenario. 

Parent item has two components, B and C. 

Cost roll wasn't done, so cost in coming from the parent item on the item branch record.  The item branch has a cost of $10.00 on the parent item. 

Component B  has a cost of $3.00

Component C  has a cost of $5.00

Sales order is entered with a W line type is created - the cost shows as $10.00

Work order is created, parts issued and completed.  Total cost on those items is $8.00.  Cardex record is written for the amount of the cost of each item.