If you only need the monthly totals, and don't want to display the details, then there's an easy solution for you that only v10 offers. In the Summary definition, set the Amount as the measure field and set the Date as the key field, and double-click the date key field item for more options. On the Matching tab, there's an option for Time Intervals. Select Calendar Month, and OK your way back to the summary.
If you need to display the details alongside the subtotals, then we'll have to look at another approach.