Being a lowly soldier in a crime family means you’re making minimum wage or less; it's the status and possibility for advancement that keeps you in it, not the money.
But when you get to be the Boss, your life changes. And just like any CEO, you take more risks, you have more responsibilities, and you probably work longer hours, Now you're getting a cut of all the businesses you're involved in: gambling, extortion, drug trafficking, neighborhood "protection" funds, and wind energy. (Hey, whatever else you say about it, the Mob has adapted to the times.)
A mob boss can "earn" an unlimited amount of money; after all, money is what the Mob is about. But can you, as the Boss, display your wealth? Flaunt it? You may want to do that outside of the city you’re working. You don't want to give the cops any cause to pick you up, you don't want the I.R.S. on your tail, and you never know which subordinate wants your job. Good thing you have several houses you can go to at any time; in your home city, Palm Beach, Marin Country, Parma, and London. And actually, Nevis Island is starting to look really good ever since the Swiss banks started getting jumpy about foreign accounts.