Subtracting Aggregate Fields In A Report

I'm trying to create an expense report based off two tables. I have a table for Bouts, and a table for Bout transactions. In the Bout transaction table there is a field that specifies whether the transaction is an expense or a revenue.

I'm trying to create a report that will split the transactions out by expenses and revenues, then in the summary show the total income, (income - expenses.) I have the report formatted, but I'm not sure how to get the grand total.

I currently have the report laid out as such:

Bout Name

Revenue

Sample Revenue 1 $100
Sample Revenue 2 $50

Revenue Total: $150

Expenses

Sample Expense 1 $100

Expense Total: $100

Net Total: (I would like to subtract the 150 from 100 here)