Rockstar has been putting a stop to those who have been trying to make a lot of money. They have done this by way of updates, which have cut down the income of people by around 50. They did this by stopping people from repeating missions and this has left people looking for ways to make cash quickly. Rockstar wants players to explore other missions, which were seen to be negative by those who played as they said that Rockstar wanted people to spend real cash in the game.
However players soon found ways of making fast cash even with the efforts of Rockstar. It was said that gamers could farm money and there is money to be found under the water if you head to the west of Los Santos. Gamers can get the money there are then change characters. When they switch back they can get the money all over again and if you do this as many as 80 times then you make $1 million in cash. Of course it can get a bit boring doing it over and over again.
You can sell cars to Simeon that you have stolen but try not to do damage to them as this will lessen the value. You can also pick up money fast by stealing from an armoured car. This way you can get as much as $5000.
You can also take to cheating and get yourself around $5 million in one hour. However the downside to cheating is that you lose your trophies and achievements for that session of the game. To get them back you have to reset it. There is a video that shows you how to do the cheat.