@alien4u Yeah, I have had similar problems before. You could POSSIBLY do a transaction search, searching all Purchase Orders or Item Receipts (therefore capturing purchase price), and only include items that have a inventory on hand >0.
Then do a formula column with a CASE statement that checks costing method. Then assigning the appropriate value. Then group all items together and sum the formula column.
In theory, this might work, but admittedly, I haven't run this in practice.