SELECT model FROM
(--таблица и модель и макс. цена
SELECT model,price FROM PC WHERE price IN (SELECT MAX(price) AS pr FROM PC)
UNION
SELECT model,price FROM Printer WHERE price IN (SELECT MAX(price) FROM Printer)
UNION
SELECT model,price FROM Laptop WHERE price IN (SELECT MAX(price) FROM Laptop)
) AS maxis
WHERE price=(SELECT MAX(price) FROM 
(--та же самая таблица только мне нужно отсюда макс цену из цен.
SELECT model,price FROM PC WHERE price IN (SELECT MAX(price) AS pr FROM PC)
UNION
SELECT model,price FROM Printer WHERE price IN (SELECT MAX(price) FROM Printer)
UNION
SELECT model,price FROM Laptop WHERE price IN (SELECT MAX(price) FROM Laptop)
) AS Maxis
)--конец подзапроса в пункте where