Since Rockstar put a stop to people making lots of money as the update they launched cut incomes by around 50%, by stopping repeated missions, players have been looking for new ways of making fast cash. It seems that Rockstar wanted to get gamers to explore other missions, which were seen as negative by those playing and gamers accused Rockstar of wanting to push people into spending real money on the game.
Gamers did find ways of making fast cash though despite the efforts of Rockstar. Gamers can “farm” money and money is there for the taking underwater to the west of Los Santos. You can take the money and then change your character, then go back to the original character. When you do this money keeps reappearing. You can keep doing the character switch and if you do it 80 times, this will earn you $1 million.
You can sell stolen cars at Simeons, however bear in mind that this doesn’t apply to all cars. Simeon will hand you a list and you need to find cars from that list. You can steal the car and hand it over to him but be careful not to damage it or you wont get as much money. You can get cash by robbing the armoured trucks and get your hands on as much as $5000.
You can also cheat and get $5 million in around one hour of playing. Bear in mind if you choose to cheat you will lose the option to get achievements and trophies for that session and will have to reset the game. If you do want to go ahead and cheat, check out the video which shows you how to get $5 million.