Had exactly same issue. If final price was ending with .50 or lower, it would drop the decimal part, and if .51 and higher, it would keep it.
This started happening as we updated from magento CE 1.9.0.1 to 1.9.2.4.
After spending a day digging, I found that it happens in mysql. Portion of the sql code that produces error:
SELECT
`cppt`.`customer_group_id`,
`cppt`.`product_id`,
CASE
WHEN IFNULL((@group_id), 'N/A') != cppt.grouped_id THEN @price :=
CASE `cppt`.`action_operator`
WHEN 'to_percent' THEN cppt.price * cppt.action_amount/100
WHEN 'by_percent' THEN cppt.price * (1 - cppt.action_amount/100)
WHEN 'to_fixed' THEN IF((cppt.action_amount < cppt.price), cppt.action_amount, cppt.price)
WHEN 'by_fixed' THEN IF((0 > cppt.price - cppt.action_amount), 0, cppt.price - cppt.action_amount)
END
WHEN IFNULL((@group_id), 'N/A') = cppt.grouped_id AND IFNULL((@action_stop), 0) = 0 THEN @price :=
CASE `cppt`.`action_operator`
WHEN 'to_percent' THEN @price * cppt.action_amount/100
WHEN 'by_percent' THEN @price * (1 - cppt.action_amount/100)
WHEN 'to_fixed' THEN IF((cppt.action_amount < @price), cppt.action_amount, @price)
WHEN 'by_fixed' THEN IF((0 > @price - cppt.action_amount), 0, @price - cppt.action_amount)
END
ELSE @price := @price -- <= THIS LINE CAUSES decimal into string conversion with rounding
END AS `rule_price`,
`cppt`.`from_date` AS `latest_start_date`,
`cppt`.`to_date` AS `earliest_end_date`,
CASE
WHEN IFNULL((@group_id), 'N/A') != cppt.grouped_id THEN @action_stop := cppt.action_stop
WHEN IFNULL((@group_id), 'N/A') = cppt.grouped_id THEN @action_stop := IFNULL((@action_stop), 0) + cppt.action_stop
END,
@group_id := cppt.grouped_id,
`cppt`.`from_time`,
`cppt`.`to_time`
FROM `catalogrule_product_price_tmp` AS `cppt`
ORDER BY
`cppt`.`grouped_id` ASC,
`cppt`.`sort_order` ASC,
`cppt`.`rule_product_id` ASC;
The line that was causing the error was: @price := @price
The solution is to edit the file app/code/core/Mage/CatalogRule/Model/Action/Index/Refresh.php, in method _calculatePrice() around line 440:
$byFixed => $this->_connection->getCheckSql(
new Zend_Db_Expr('0 > @price - cppt.action_amount'),
new Zend_Db_Expr('0'),
new Zend_Db_Expr('@price - cppt.action_amount')
),
)
)
),
'@price := @price' // <-- the culprit
);
}
replace with:
$byFixed => $this->_connection->getCheckSql(
new Zend_Db_Expr('0 > @price - cppt.action_amount'),
new Zend_Db_Expr('0'),
new Zend_Db_Expr('@price - cppt.action_amount')
),
)
)
),
'@price' // remove ':= @price' to fix rounding bug
);
}
