This is what I could come up with quickly but I haven't tested thoroughly This month -

CASE WHEN MONTHS_BETWEEN(LAST_DAY(SYSDATE), LAST_DAY({trandate})) = 0 THEN {amount} ELSE 0 END

Last month -

CASE WHEN MONTHS_BETWEEN(LAST_DAY(SYSDATE), LAST_DAY({trandate})) = 1 THEN {amount} ELSE 0 END

Two months ago -

CASE WHEN MONTHS_BETWEEN(LAST_DAY(SYSDATE), LAST_DAY({trandate})) = 2 THEN {amount} ELSE 0 END

etc...