Whilst not being as essential as in, say, GTA, cash is pretty useful in Watch Dogs 2 and is needed for a couple of the achievements. There are three main ways to boost your cashflow in game.
The Advanced Profiling skill in the Social Engineering tree will allow you to identify people with large bank accounts that you can hack for anywhere between $500-$700 each. If you keep an eye out for the blue boxes that signify them as you’re out and about in the world, and hack them as you go, you’ll build a substantial bank account before you know it.
Almost any vehicle you steal will have something valuable in the glove box that you can steal as a little bonus. Sell these at a pawn shop to get some extra cash.
There are valuables and money bags out in the world that you can find and pick up. They’ll appear on both your mini-map and your world map, and they are everywhere. The big bags of cash tend to be in restricted areas, but usually aren’t particularly difficult to get to. The smaller pick-ups are anywhere between a few hundred and a couple of thousand dollars each, whilst the bigger bags are usually around $18k-$20k each, meaning if you get a few of them you can afford almost anything you want in no time.