http://sqlfiddle.com/#!9/8c7fc59/11


SELECT SUM(amounts_left*price)/SUM(amounts_left) as eBuyIN, SUM(amounts_left), instrument_id FROM transactions as t WHERE 1 GROUP BY currency_id,broker_id,portfolio_id,instrument_id;

eBuyIN  SUM(amounts_left)   instrument_id
89.600000000000000000000000 6.0000000000    42765
1.325909604519774011299435  2655.0000000000 42807
1.090000000000000000000000  480.0000000000  42811
8.930000000000000000000000  57.0000000000   42812
0.980000000000000000000000  501.0000000000  42813
150.000000000000000000000000    12.0000000000   71523
89.600000000000000000000000 6.0000000000    71552


SELECT
    SUM(price)/SUM(mleft), SUM(mleft), `instrument_id`
FROM(
    SELECT
        `instrument_id`, currency_id, broker_id, portfolio_id,
        amounts_left AS mleft,
        amounts_left * price * (
             SELECT `rate`
             FROM `currency_exchanges`
             WHERE `from_currency_id` = t.currency_id
               AND `to_currency_id` = 2
               AND `date` <= t.created
             ORDER BY `date` DESC LIMIT 1
        ) AS price
    FROM `transactions` AS `t`
    WHERE `action` = 1 AND amounts_left > 0
      AND `portfolio_id` = 128
) a group by instrument_id, currency_id, broker_id, portfolio_id



 SUM(price)/SUM(mleft)  SUM(mleft)  instrument_id
89.60000000000000000000000000000    6.0000000000    42765
1.32590960451977401129943502825     2655.0000000000     42807
1.09000000000000000000000000000     480.0000000000  42811
8.93000000000000000000000000000     57.0000000000   42812
0.98000000000000000000000000000     501.0000000000  42813
150.00000000000000000000000000000   12.0000000000   71523
89.60000000000000000000000000000    6.0000000000    71552


 WITH cte AS
(
         SELECT   `instrument_id` ,
                  row_number() OVER(partition BY t.currency_id,t.broker_id,t.portfolio_id,t.instrument_id ORDER BY amounts_left DESC )                                                         AS rownumber,
                  sum(amounts_left) OVER(partition BY t.currency_id,t.broker_id,t.portfolio_id,t.instrument_id ORDER BY amounts_left DESC rows BETWEEN CURRENT row AND      1 following)       AS amountsleftt,
                  sum(amounts_left*price) OVER(partition BY t.currency_id,t.broker_id,t.portfolio_id,t.instrument_id ORDER BY amounts_left DESC rows BETWEEN CURRENT row AND      1 following) AS pricet
         FROM     currency_exchanges                                                                                                                                                           AS ce
         JOIN     transactions                                                                                                                                                                 AS t
         ON       from_currency_id = t.currency_id
         AND      to_currency_id = 2
         AND      ce.date <= t.created
         AND      action = 1
         AND      portfolio_id = 128 )
SELECT pricet/         amountsleftt,
       amountsleftt AS total,
       `instrument_id`
FROM   cte
WHERE  rownumber = 1;


priceT/amountsleftT Total   instrument_id
89.600000000000000000000000 12.0000000000   42765
1.230000000000000000000000  1000.0000000000 42807
1.090000000000000000000000  960.0000000000  42811
8.930000000000000000000000  114.0000000000  42812
0.980000000000000000000000  1002.0000000000 42813
200.000000000000000000000000    12.0000000000   71523
89.600000000000000000000000 12.0000000000   71552