SELECT b.full_name,
  SUM((SELECT price FROM prices p WHERE p.date < a.date AND p.PRODUCT_ID = a.PRODUCT_ID ORDER BY date DESC LIMIT 1) * a.quantity) AS TOTAL
FROM
  historysales a
  INNER JOIN customer_w b ON (a.customer_id = b.customer_id)
WHERE
  `date` BETWEEN ('2006-01-01') AND ('2008-01-01')
GROUP BY b.full_name,DATE_FORMAT(a.`date`, '%b-%Y')
order by date