We want to pop-up a screen to show the total value of PO before saving the PO so that user can decide whether to save the PO or discard it. What is the best way?

Try using EXIT_SAPLEINS_001. It is entered during the saving operation of a PO. You can review the PO, present a popup dialog using one of the POPUP_* functions enabling the user to decide either to save or abort the PO.