Maksim Tsisar

Используйте вложеные запросы

SELECT Id, MIN(Price) FROM
(
SELECT post_id AS Id, meta_value AS Price FROM wp_postmeta WHERE meta_key = '_price' AND post_id IN (SELECT post_id FROM wp_postmeta WHERE meta_key = 'convert_to_rub' AND meta_value = 0)
UNION
SELECT post_id AS Id, meta_value * {КУРС_ВАЛЮТЫ} AS Price FROM wp_postmeta WHERE meta_key = '_price' AND post_id IN (SELECT post_id FROM wp_postmeta WHERE meta_key = 'convert_to_rub' AND meta_value = 1)
)
GROUP BY Id