The easiest way to make money is, buy a nightclub and let all your businesses run passively. Put your character in the chair of the security camera monitor and your session won't time out even if your controller does. If you plug the controller in to the usb port, the controller won't...
then leave the same way you got in. Get to the main gate with a mixture of stealth and kill anyone you feel you need to. After the cutscene of you leaving, run straight towards the red marker on the map. His back is to you so take ...