EventListener/ProductDoctrineEventSubscriber.php
protected function calculateAttributePrice(VariantInterface $variant, $amount)
    {
        $modifierType  = $variant->getModifierType();
        $modifierValue = $variant->getModifierValue();
        
        switch ($modifierType) {
            case '+':
                $amount = $amount + $modifierValue;
                break;
            case '-':
                $amount = $amount - $modifierValue;
                break;
            case '%':
                $amount = $amount * ($modifierValue / 100);
                break;
        }
        
        return round($amount, 2);
    }
protected function calculateAttributePrice(VariantInterface $variant, $amount)
    {
        $modifierType  = $variant->getModifierType();
        $modifierValue = $variant->getModifierValue();
        
        switch ($modifierType) {
            case '+':
                $amount = $amount + $modifierValue;
                break;
            case '-':
                $amount = $amount - $modifierValue;
                break;
            case '%':
                $amount = $amount * ($modifierValue / 100);
                break;
        }
        
        return round($amount, 2);
    }
Calculates new amount for attribute

@param VariantInterface          $variant
@param                           $amount

@return float