Hi SDN,
I am not a MM consultant (I am a finance user) so I hope to get some clarification from your bright minds 
Our requirement is pretty much that of a classic subcontracting scenario in SAP wherein we purchase packaging material from Vendor A after which we send it to Vendor B to consume. Lastly, GR the Finished Good from Vendor B.
Here are what we currently do:
1. Place a PO to the vendor with item category L. This item category causes the system to automatically derive the processing components required for the FG.
2. The PO price covers only the FG cost, excluding the packaging. This is the price that we pay to the vendor. The PO is saved and sent.
3. We transfer the packaging material from our warehouse to the "stock at subcontractor", movement type 541.
4. When the packed material is delivered home, we report the goods receipt on the PO above. This causes the stock of the packaging material at the subcontractor to be reduced (issued) automatically using mvt type 543.
5. The vendor invoice which is posted with the reference to the PO above covers only the cost of the FG, excluding the packaging material.
We are currently valuating all materials using standard price.
This currently results in undervalued Finished Good inventory (it excludes the packaging material cost). If we are to increase the cost estimate of the FG, this will result in massive PPV due to the difference between the cost estimate (includes packaging cost) and purchase price (does not include packaging cost)
Any suggestions you can think of to get around this?