Monitoring your income and expenses within your country is vitally important. Much of the time your cabinet ministers will handle much your economy for you but it's their results that you should keep an eye on. An example of why you'll want to watch is when you want to increase your military. For this, you'll need to know if your economy can handle the increased production costs, increased salaries and training costs and added maintenance costs.
In the Treasury Department you will find two key reports for income and expenses. Here are the areas of your government that fall under these categories:
Income --- Income Tax, Sales Tax, Corporate Tax, Import Duty, Export Duty, Employment Tax, Property Tax, User Fees, Pension Tax, Immigration, Foreign Aid, Domestic Sales, Loan Principle, Loan Interest, Trade Surplus
Expenses --- Military Spending, Units Building, Preparedness, Intelligence, Research, Production, WM Membership, Foreign Aid, Social Services, Construction, Maintenance, Loan Repayment, Interest, Trade Deficit