[Sham]

Код:

UPDATE
  `table`
SET
  `price` = 0.1 * `price` + `price`
WHERE
  `cat_id` = 1

для проверки сначала можно select вывести

Код:

SELECT
  `price`,
  (0.1 * `price` + `price`) AS `newprice`
FROM
  `table`
WHERE
  `cat_id` = 1