The first thing to know about money is this- it's really not that important. Maybe there's a life lesson there, but particularly MAFIA:III you'll ind it's easy to rake up more cash than you really need.
The main expense in Mafia 3 is weapons, and while it's nice to have your guns of your own, virtually every mission you undertake is going to involve knocking over dozens of people with guns and ammo, all free for the taking. Plus, many of the best weapons (such as the silenced guns) are given to you through story progress, rather than purchase. If you can live without having your own private arsenal, you'll save tons of dough for what really matters - upgrades.
Why spend thousands of dollars on a slightly more accurate pistol, when you could spend thousands to make every pistol you touch more accurate? Once you get access to the Weapons Van, you'll be able to buy upgrades for Lincoln himself. But since most of these upgrades only become available for purchase when you've advanced to certain point in the storyline, you have plenty of time to build up your cash reserves between upgrades. You can also prioritize purchases here- if you always rock a pistol and machinegun, there's no hurry to purchase rifle upgrades.
That said, it's better to have too much money than not enough. Lincoln will end up with two main revenue streams over the course of the game. The first: money found lying around. The streets of New Bordeaux ain't exactly paved with gold, but there sure are a lot of cash bags and dollars stacks lying about. Especially in mob hideouts and safehouses, which you'll spend a lot of time sneaking through and busting up. Some of these additions to your wallet might seem small, but they add up fast- pocket all the cash you see to secure a comfortable financial future.
The second stream is something called kickbacks. Once you advance to the point where you have underbosses working for you, they'll kick up a percentage of their take based on how much they make, and how much they like you. This continues to roll in throughout the game, raising your bank balance ever higher. You can increase the flow by improving your bosses' operations, and doing mission to make them more loyal to you, but the flow never stops. If really need more money, just wait for it.
Life can be cheap in New Bordeaux, but death gets downright expensive- everytime you die, you lose half of the cash you're carrying. Avoid this perfidious death tax by stashing your money in Sammy's safe whenever you're in the neighborhood, or about to do something dangerous. Once you reach a point where you're recruiting underbosses, make a point of getting on Vito's good side as quickly as possible. His consigliere perk allows you to deposit money at virtually any time, and with the rigth upgrades, this same consigliere will pick up your kickback cash as well, saving you long trips out to see each of your underbosses in turn.
Follow these tips, and when you do come across something you absolutely must buy, you'll find it barely makes a dent in your fortune.