code: protected function refreshProductVariantSellPrice(VariantInterface $variant)
      {
          $product               = $variant->getProduct();
          $sellPrice             = $product->getSellPrice();
          $grossAmount           = $this->calculateAttributePrice($variant, $sellPrice->getGrossAmount());
          $discountedGrossAmount = $this->calculateAttributePrice($variant, $sellPrice->getDiscountedGrossAmount());
          $taxRate               = $product->getSellPriceTax()->getValue();
          $netAmount             = TaxHelper::calculateNetPrice($grossAmount, $taxRate);
          $discountedNetAmount   = TaxHelper::calculateNetPrice($discountedGrossAmount, $taxRate);
          
          $productAttributeSellPrice = $variant->getSellPrice();
          $productAttributeSellPrice->setTaxRate($taxRate);
          $productAttributeSellPrice->setTaxAmount($grossAmount - $netAmount);
          $productAttributeSellPrice->setGrossAmount($grossAmount);
          $productAttributeSellPrice->setNetAmount($netAmount);
          $productAttributeSellPrice->setDiscountedGrossAmount($discountedGrossAmount);
          $productAttributeSellPrice->setDiscountedTaxAmount($discountedGrossAmount - $discountedNetAmount);
          $productAttributeSellPrice->setDiscountedNetAmount($discountedNetAmount);
          $productAttributeSellPrice->setValidFrom($sellPrice->getValidFrom());
          $productAttributeSellPrice->setValidTo($sellPrice->getValidTo());
          $productAttributeSellPrice->setCurrency($sellPrice->getCurrency());
      }
docs: Recalculates sell prices for single product attribute
  
  @param VariantInterface $variant