Lester has some work for Franklin.
After the story mission “Fame or Shame” Lester will call Franklin and offer some work. His tasks will be related to the stock market — the only opportunity to make really big money in Grand Theft Auto V. But you need to follow the next three pieces of advice to do it.
First, before the first assassination, you need to perform as many missions of strangers and freaks and random events, that bring considerable money, as possible. The exception is “Hitch Lift 1” because it also related to the stock market. “Bike Thief City 1” can be performed, because there is no real stock market game — you just sell stocks at a fixed price.
Second, perform the second assassination only when protagonists will get the money from the last heist.
Third, refrain from expensive acquisitions before you get the profit from the last assassination. Don’t buy properties, vehicles, weapons, and modifications for the last two without necessity. Buying of garages is admissible. Buying of slips, helipads, and hangars is better to put off.