Purchase Orders total amount



I am trying to find a way to calculate total of extended cost entered during Purchase Order entry in P4310 and show it in header to users as they enter new lines.
The idea is to provide users with a running total as they add lines.
Is this possible? If not what are the alternatives. Example in P4210 we have a order summary form option that we can show before order acceptance. Is something similar possible in PO (P4310).

Thanks for help.