On this Empire of Sin guide page, you will learn more about gang finances. Here you will learn what to do to make your empire profitable.
The state of your funds is visible at any time of the game - just look at the bar at the top of the screen. That way you are able to check how much cash you have and what your monthly income (or loss) is.
In EoS, making money is not a big challenge. The easiest way to do that is to take over buildings that don't belong to any of the gangs. Initially, when you're low on cash, you can simply rob the buildings. However, it is more profitable to turn them into rackets. This will make them generate income regularly. Brothels are the safest of the businesses. They generate small but steady profits. They also do not require a supply of alcohol.
Among the more profitable are speakeasies and casinos. In the illegal bar's case, you should also have a brewery that will keep it stocked with booze. A casino, on the other hand, carries the risk of financial losses caused by one of the clients winning.
You have to remember that each neighborhood has a specific pool of money that you can earn there. You'll know how much by looking at the neighborhood panel. If you're already taking 100% of the revenue, continuing the expansion in a given neighborhood is pointless.
Additional funds can also be raised by selling items acquired during combat. You can exchange surplus goods for cash thanks to a black market trader or when dealing with other bosses.