Hi Nayrod!
You can do this with the update_field function.
https://www.advancedcustomfields.com/resources/update_field/
update_field($selector, $value, [$post_id]);
$selector is your price/per kilo field name
$value is calculated price
$post_id is current product ID
You can use a function in your function.php file to calculate the price ($value).