Vendor Validate

The Direct Unit Cost gets calculated from Purchase Price table. Which in your case is empty.
From I understand you want the price of the Sales Order be the price on the PO? this this case you not making any profit?
Is this a drop shipment?

Anyways If you want to change the cost, you can do it in the following function.

In requisition Line table function GetDirectCost(CalledByFieldNo : Integer)

You can change it to grab it from related sales line

Here is the code

IF "Replenishment System" = "Replenishment System"::Purchase THEN BEGIN
  PurchPriceCalcMgt.FindReqLineDisc(Rec);
  PurchPriceCalcMgt.FindReqLinePrice(Rec,CalledByFieldNo);
  //Start 001
  IF ("Direct Unit Cost" = 0) AND (CalledBYFieldNo = FieldNo("Vendor No.")) and ("Sales order No." <> '') then begin
    if SalesLine.get(SalesLine."Document type"::Order,"Sales Order No.","Sales Order Line No.") then begin
       "Direct Unit Cost" := SalesLine."Unit Cost (LCY)";
    end;
  end;
  //End 001
END;