It is easy to manage one's monthly budget by marking tasks that need to be paid with "money" tag and creating a smart list with the following criteria:
"tag:money and dueAfter:02/00/2014 and dueBefore:03/01/2014", for example for january, 2014.
Of cource you can go deeper and diversify expenditures by adding more complex tagging.

Have a nice RTM usage!