code: 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;
      }
docs: Compares the units regarding to their price.
  
  @param StockUnitInterface $u1
  @param StockUnitInterface $u2
  
  @return int