protected function compareStockUnitByPrice(StockUnitInterface $u1, StockUnitInterface $u2)
    {
        $u1HasPrice = 0 < $u1->getNetPrice();
        $u2HasPrice = 0 < $u2->getNetPrice();

        if (!$u1HasPrice && $u2HasPrice) {
            return 1;
        }
        if ($u1HasPrice && !$u2HasPrice) {
            return -1;
        }
        if ($u1->getNetPrice() != $u2->getNetPrice()) {
            return $u1->getNetPrice() > $u2->getNetPrice() ? 1 : -1;
        }

        return 0;
    }