How to derive Planned costs for OSP operation in Fusion

Summary:

How do we derive planned costs for an OSP Operation in Fusion
We operate in actual costing environment, and this is the problem statement
We know that in a work definition, for regular Inhouse operations we can assign planned resource usage hrs. and derive budgeted hours/planned cost
But for OSP process the system does not allow resource assignment, in this case how do we arrive at estimated cost for OSP process. Because when we process and invoice a OSP PO it is the actual cost incurred at the end of the process for OSP service but there is no planned cost to compare with.