GetPriceWithDiscount:

$dbPrice = CPrice::GetList(
        array("QUANTITY_FROM" => "ASC", "QUANTITY_TO" => "ASC", "SORT" => "ASC"),
        array("PRODUCT_ID" => $ID),
        false,
        false,
        array("ID", "CATALOG_GROUP_ID", "PRICE", "CURRENCY", "QUANTITY_FROM", "QUANTITY_TO")
    );
  while ($arPrice = $dbPrice->Fetch())
  {
    $arDiscounts = CCatalogDiscount::GetDiscountByPrice(
            $arPrice["ID"],
            $USER->GetUserGroupArray(),
            "N",
            SITE_ID
        );
    $discountPrice = CCatalogProduct::CountPriceWithDiscount(
            $arPrice["PRICE"],
            $arPrice["CURRENCY"],
            $arDiscounts
        );
    $arPrice["DISCOUNT_PRICE"] = $discountPrice;

    echo "<pre>";
    print_r($arPrice);
    echo "</pre>";
  }