What you want is called a summary report.
You can make a list view report that uses a sort to group your records first by client and then by the billing status field. Using sub-summary parts as a place for summary fields that compute your subtotals for each.
Here's a link to a simple tutorial on setting up summary reports that you may find useful:
Creating Filemaker Pro summary reports--Tutorial
Thanks!! I'm going to get on that right now.