Primary Location 08 Cost Update

Hi,

Can somebody help me with the following issue?

We are on 9.1 and I am looking to update the cost for 08 cost method in the primary location even though I am using a lot controlled item. So, when I am trying to enter a Purchase Order in P4310, the Unit Cost field should be populated with the last cost used for that item, the cost that has been used at the receipt, P4312. Currently, my cost methods are 06 Lot for Sales/Inventory and 08 Purchase Based for Purchasing and when I receive an order, it creates me a lot number which stores the last cost, but the