Yeah, all of the best weapons and armor you can buy costs more than you'll make in a single playthrough of the campaign. And we're not resorting to buying Hack Points, are we? Of course not, we're real assassins. (Not actually.)
So here's a few short tips to kickstart your lucrative career in overdrive.
Don't spend your money on weapons. Once you've picked a weapon of choice, you can get by with 1000 livre version of that weapon. Upgrade its damage with some Creed Points and you'll be good to go. The game will throw one of the best weapons at you once you finish the story campaign.
Do spend your money on Cafe Theatre improvements and Social Clubs. Just like real life, property is everything. Every improvement to your home base and new social clubs means more cash flowing into your coffers. Buy the first few Cafe Theatre improvements and complete the available missions there to upgrade even more.
Choose the right skills. If you want money, one of the primary skills you need to pick up first is lockpicking. When a new version is available (Sequences 2, 5, and 9) buy it. not only does this help with unlocking doors in missions, but unlocked entrances lead to treasure chests. In addition, the red chests on your map are all locked chests. No lockpicking? You're leaving money on the table.
Go hard or go home. The diamond system is everything. The enemies in four- and five-diamond districts are harder to kill if you're not equipped, but they give more money. The payout for five-diamond missions is far better.
Play well with others. Co-op missions are fine for Sync Points and gear, but if you want money, Heists are where it's at. You receive a specific pay-out for finishing a Heist but there's also a huge bonus pay-out. This bonus starts at a specific amount and goes down when you do non-stealthy things. Stay stealthy and reap the rewards!!
A better way to grind. Heists are great, but they're time-consuming and you're not guaranteed to finish, unless you have a solid, consistent crew. If you want to avoid that mess, the best way to make money in Assassin's Creed Unity is through Helix Rift missions. These missions - SPOILER - are unlocked at specific points in the game and take you to other time periods. The first mission is always a basic run-through of the entire Rift and the second mission is a Data Harvest. The latter mission is the moneymaker.
Data Harvests reward good money for just completing them and they give really good money if you collect enough bonus data. Plus, once you finish a Helix Rift Data Harvest once, it unlocks a ghostly guide. Follow the guide and most of the time you'll hit the bonus data level. The latter maps like Medieval - Battlefield (my favorite) offer 8,000 livres as a basic pay-out and nearly double that for hitting the bonus.
The best part is these missions are only 3 minutes long (plus a couple of seconds to escape). Either you get paid or your fail. Once you have them down, Helix Rifts are easy, consistent money. If you can hit that bonus on a regular basis, you won't find better money in the game. Get to grinding, assassin.