One of the easiest ways to earn cash in Mafia III is to kill your enemies. Like you, they’re gangsters, except they aren’t in it to avenge their murdered kin. They want money and power, just like all of the other people you hate in the game, and they like to carry that money and power on their person. You’ll get a bit of cash with each and every gangster you kill, so you may as well kill every last one you see.
Of course, with this being the easiest way to earn some much needed cash in New Bordeaux you shouldn’t expect to make a whole lot. While this is a means to make enough to keep your weapons topped off with ammo and things of that nature, you won’t become a rich man shooting basic enemies.