Sum Data By Group

how to write a macro that will automatically input a sum formula under a range of numbers in my file. An example is below. Every month I get this report and I then true up the numbers which is why I need the sum formula at the bottom of each category. The problem is every month is not necessarily the same length, but the report is a few thousand lines long, so a macro to do it instead of manually doing
Example:

Revenue
Revenue source 1
Revenue source 2
Total Revenue--------SUM(all revenue cells)

Payroll
Tax
Incentives
Vacation
Etc
Etc
Total Payroll---------SUM(All Payroll cells)

Warehouse
Supplies
Maintenance
Etc
Total Warehous-----Sum(All Warehouse)