Why not simply value the goods receipts as I mentioned in another post? This can be done on the basis of an already received supplier invoice or the prices indicated on the purchase order, which normally are confirmed by the supplier.
If the invoice contains only part of the quantity received, then only the actual quantity invoiced is valuated at the invoice price at goods receipt. The remaining quantity is valuated at the purchase order price.
I know that at least SAP handles goods receipts that way. (See their BLOG entry about this here)