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.