Inventory valuation report

When I run an inventory valuation report, it seems to pull the cost associated for each item from the item properties, not from costs entered on purchase orders. For our inventory, we have items we bough at booking prices early in the season and items we bought on regular prices later in the season. We need to have the item properties reflect the higher cost but that inflates our inventory valuation report. Is there a way to have the report draw the costs paid for each item by PO instead of the cost entered when we created each item? Otherwise that report really is not accurate at all.