refund_code    refund_amount    month_group
-----------    -------------    -----------
ref1               10           january
ref2               20           january
ref3               30           january
ref1               40           february
ref2               50           february
ref3               60           february

refund_code    refund_amount    month_group    month_total
-----------    -------------    -----------    -----------
ref1               10           january                 60
ref2               20           january                 60
ref3               30           january                 60
ref1               40           february               150
ref2               50           february               150
ref3               60           february               150

SELECT mr.month_group,
    bd.transaction_code AS refund_code,
    SUM(bd.extended) AS refund_amount
FROM   billing_details AS bd
    LEFT JOIN monthly_ranges AS mr
        ON ( bd.entry_date BETWEEN mr.start_date AND mr.end_date )
WHERE  bd.transaction_code IN ( 'REFPRI', 'REFSEC', 'REFPT', 'REFREQPRI' )
    AND bd.entry_date >= '2012-01-05'
GROUP BY mr.month_group, bd.transaction_code
ORDER BY mr.month_group, bd.transaction_code

SELECT mr.month_group,
    SUM(bd.extended) AS refund_amount
FROM   billing_details AS bd
    LEFT JOIN monthly_ranges AS mr
        ON ( bd.entry_date BETWEEN mr.start_date AND mr.end_date )
WHERE  bd.transaction_code IN ( 'REFPRI', 'REFSEC', 'REFPT', 'REFREQPRI' )
    AND bd.entry_date >= '2012-01-05'
GROUP BY mr.month_group
ORDER BY mr.month_group

SELECT m.refund_code, m.refund_amount, m.month_group, t.month_total
 FROM (your refund query above) m
 JOIN (your total query above) t
   ON m.month_group = t.month_group