Infinite money
Late in the game's campaign, the "An Eye For Eye" main operation will become available, with "A Real Dog Fight" sub-mission as its fifth main component. During the "A Real Dog Fight" sub-mission, you will go down a large freight elevator. When it reaches the proper floor, proceed to the indicated in-game mission area with the double doors, and hack the camera to see a room full of people with a man and woman standing next to each other giving a speech. The man is Pablo The Skinner. Point the camera at him and hack him to get approximately $18,000.
Then, simply display your phone, select "Game Options", choose "Reload Last Checkpoint", and repeat the process as many times as desired. When the checkpoint loads, you can run into the back left corner of the elevator and hack Pablo The Skinner to do this even quicker. You can get over $1 million in less than an hour using this glitch. Note: This glitch was performed on an unpatched version of the game. It will probably eventually get patched. To avoid not being able to use this exploit, either do not install new patches before using this exploit or delete the patches. You can avoid patches being installed by disconnecting from the internet until you are ready for the game to install new patches.