Perhaps the best way to earn money in the game is through rackets. You assign underbosses to a racket, and then you can periodically visit them to collect whatever they earned for you during your absence. It's important to visit them often, though, since there are limits to how much money they'll keep safe on your behalf. It's also worth progressing the story missions, since completing them will raise the cap and also give you new ways to build your fortune. In particular, try to play until at least the point when you meet Donovan, who you will find on a bench in Barclay Mills. And once you have put away $140,000, you can also hire someone to collect racket money on your behalf, so that you can spend your time and energy elsewhere.