Вземете минималната цена за всеки тип продукт с конвертиране на валута

    SELECT a.id, a.price*ISNULL(s.to_aud,1) as minprice
FROM
(
 SELECT type_id, min((price+shipping)) as minprice
 FROM products
 GROUP BY type_id
) AS b INNER JOIN products as a on a.type_id = b.type_id and (a.price+a.shipping) = b.minprice
Inner join sellers s on s.id = a.seller_id
ORDER BY price