Cost of keeping open and no longer needed PO's?

The Purchasing manager at my company has asked me if there is any data on the cost of keeping open and no longer needed PO's in Oracle.  He is attempting to gather business support for a mini project to finally clean up the hundreds and hundreds of PO's that have not been properly managed over time.  Does anyone have any suggestions on how to apply a cost to that?  I'm not necessarily looking for a dollar amount, just ways in which keeping old or bad data bogs down the system.

We are on 12.1.3 and use Purchasing with workflow approvals.  We do not use requisitions.