public function GetTotalDiscountValue()
    {
        $iCurrentPosition = $this->getItemPointer();
        $this->GoToStart();

        $dTotalDiscountValue = 0;

        while ($oItem = &$this->Next()) {
            $dTotalDiscountValue += ($oItem->dPriceTotal - $oItem->dPriceTotalAfterDiscount);
        }
        $this->setItemPointer($iCurrentPosition);

        return $dTotalDiscountValue;
    }