SELECT
  min(value) AS min,
  max(value) AS max
FROM `prices`
WHERE exists(SELECT *
             FROM `variants`
             WHERE `prices`.`variant_id` = `variants`.`id` AND `pack_quantity` = (SELECT max(pack_quantity)
                                                                                  FROM `variants` AS `v2`
                                                                                  WHERE
                                                                                    product_id = `variants`.product_id
                                                                                    AND `available_quantity` > '0')) AND
      `currency` = 'RUB'
LIMIT 1