The biggest trick with Grand Theft Auto 5 is to save the Lester Missions for last. Go thru all of the heist missions to make a good amount of money (between 15-40 million depending on your choices), then use that money tospeculate on the stock market. Each of Lester’s missions will return a great percentage and you can easily make up to about 3 billion dollars after completing them. Make sure you choose the companies below for the maximum return. Buy heavily in these companies with all 3 of your characters before starting the mission.
Hotel Assassination: Beta Pharmasuticals.
Redwood Cigarette Assassination: Debonaire Cigarettes. (Sell when it tops off about 80% then put your money into Redwood Cigarettes, wait 48 hours for Redwood to jump to 300%)
Vice Assassination: Fruit.
Bus Assassination: Vapid.
Construction Assassination: Gold Coast Development