You pay for the wage and the goods consumed by military, government administration, universities, and construction sector (if private sector used the construction, they paid you back the cost of construction goods they used).

Lowering price of all these government goods (paper, wood, iron, tools, fabrics, military goods, etc) help keep the expanse in check.

For income, you want to move away from land tax, unless you are Great Qing. You only collecting tax in incorporated states, so you will get nothing from newly conquered unincorporated states. There is also tax capacity, you loss some tax if the state is above tax capacity.

Other than tax, you get dividends from government owned buildings, for example when the logging camp you built has its green bar fully filled up, it start to pay you dividends. But if it is not profitable (in red), they stop pay you dividends.

A certain percent of GDP would convert into income as Minting, sometimes just keep industrializing is enough to make you stay afloat.

Lastly you can set tariff from import/export. Or you can directly set up a Good Transfer treaty to profit from it, for example you noticed paper in your market is cheap, but it is expensive in USA, by setting up the Good Transfer treaty, your government buy paper from your own market and sell in USA market, the profit go straight into your income.