code: private function getPurchaseCost(Model\SaleItemInterface $item)
      {
          /** @var \Ekyna\Component\Commerce\Subject\Model\SubjectInterface $subject */
          if (null === $subject = $this->subjectHelper->resolve($item)) {
              return null;
          }
  
          $currency = $item->getSale()->getCurrency()->getCode();
  
          if (null !== $cost = $this->purchaseCostGuesser->guess($subject, $currency)) {
              return $cost;
          }
  
          return null;
      }
docs: Returns the sale item purchase cost.
  
  @param Model\SaleItemInterface $item
  
  @return float|null