The source of the PO cost is set on the Item Master,in the Purchase Price Level field (PPLV). If this is set to 1 or 2, the cost comes from the supplier/item price. If it is set to 3, it comes from the F4105 cost. However, if it is set to 1 or 2 and there is no supplier/item price set up, it will also come from the F4105.
In the F4105, the 08 cost will be pulled if it is the cost method specified in the Purchasing Costing Method (PCSM).
The 08 cost is automatically updated with the base cost on each PO receipt.
Hope this helps.