Cost of Raw Material

Hi,
I have a requirement as below, kindly let me know how we can map the solution in Navision 2009 SP1 to get the cost automatically.
Consumption - Raw Material (RM)
Item A (330 mm) - 1 pc. Unit of Measure – PCS

Output - Finished Goods (FG)
Item C (80 mm) FG- 4 pcs. Unit of Measure – PCS
Item B (10 mm) RM - 1 pc. Unit of Measure – PCS

Requirement
They need 4 pcs of 80mm rod for sales. Hence the above item A (RM) is cut into 5 pcs. i.e. 4 pcs of 80 mm Item C is created along with 1 pc of 10mm Item B (bi-product). Item (RM) B can be re-used for other production.

My current Solutions
Solution 1
Use consumption journal, where user can select the item B and enter the quantity in minus and cost manually.

Solution 2
Use Inventory journal - Positive adjustment, where user can select the item B and enter the quantity and cost manually.

Is there is any better way to get the cost automatically for item B? i.e. How we can get the exact cost of 10mm to Item B from Item A, after allocating the necessary cost to Item C
Customer is not willing to use Standard cost for Item B, as there is a fare chance for procurement of Item B from other suppliers

Solution 3 - buy the add-ons for the process industry that handle this situation. Process 800 is one, there are others.