The best way to make a lot of money in the story mode of GTA 5 is to complete Lester ‘s stock market assassination missions. Our Assassination Investor page details the steps required for developing your finances beyond the Billion dollar mark. The old trick of buying low and selling high still applies to GTAV. Keep an eye on the news as well to see which stocks are doing good or bad. To make a quick return, save your game and then shut off auto save. Invest all of your money into a stock that you think is on the rise. Go to your safehouse and rest without saving or wait roughly 45 seconds and check your portfolio to see your profit. Keep checking the stock until it peaks out and then sell.