This is just an idea - it need to be tried on your schema and system. Thinking that you want a grand total (sum) for certain months.
SELECT "Grand Total - June 2009 - September 2009", SUM(plan_total)
FROM (SELECT TO_CHAR(c.END_DATE,'MM-YYYY') years,
SUM (b.plan_total_lcl) plan_total,
SUM (b.actual_total_LCL) actual_total
FROM KCST_BUDGET_PERIOD_SUM a,
KCST_CURRENCY_LINES b,
KNTA_PERIODS c
WHERE a.BUDGET_ID = (SELECT z.PRJ_BUDGET_ID FROM KCRT_FG_PFM_PROJECT z WHERE z.REQUEST_ID = 43985)
AND a.currency_line_id = b.currency_line_id
AND c.period_id = a.period_id
GROUP BY TO_CHAR(c.END_DATE,'MM YYYY')
ORDER BY 1) X
WHERE to_char(Years,'MM-YYYY') between to_char('01-06-2009', 'MM-YYYY') to to_char('01-09-2009','MM-YYYY');