This is straight from another forum I go to called hoops-nation.com. Credit goes to my friend So Steezyy for putting it up there. It works 100%, I did it.
Once you unlock Trevor and you are able to switch back to Michael and Franklin again you will get these Assassination missions with Franklin, DO NOT do these missions and save them until you have already beat the game so that you have more money to invest. You will have to complete the first one though to be able to move on with the story.
To make sure you don't mess up at any time make sure to quick save before missions so you can go back if you have too.
Hotel Assassination (You are forced to do this mission in order to move on with the story): Before starting the mission with all 3 players invest in BettaPharmaceuticals, it's in the Bawsaq market if you can't find it. After completing the mission the stock will rise about 50%, might be different for everyone. After selling you can invest all of that money into Bilkinton (LCN market), I've seen people say that stock recovers, not sure how long it takes though.
Now you want to save the remaining 4 assassination missions until you have already beat the game so that you have more money to invest.
Before starting the rest of the assassination missions you want to do one of the random missions where you help a guy out and drive him to the airport. In return he gives you stock advice that will give you a 30% return. Before starting this mission invest with all 3 players in Tinkle (Bawsaq market).
To find the guy you have to use Michael and go to this location
Once you have done that you want to do all of the assassination missions
The Multi Target Assassination: Invest with all of your players in Debonaire (LCN market) before starting the mission. Once you complete the mission and the stock has peaked sell your stocks then invest all of your money in Redwood (LCN market), Redwood will recover and go up about 300%.
The Vice Assassination: Before starting the mission invest with all 3 guys in Fruit (Bawsaq market), should get about a 50% return.
The Bus Assassination: AFTER you complete the mission with all 3 characters invest in Vapid (Bawsaq market), you will get about a 100% return.
Construction Assassination: Before the mission with all 3 characters invest in Gold Coast Development (LCN market), you will get about a 80% return.
If you did all of this correct you should have between 1.9 billion and 2.1 billion.
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Me writing:
I made this a thread on its own because we always post in the main GTA V Shop thread and it might get skipped a few times, so it'd be better this way.
When you're investing the money, make sure to invest all of it. It's a guaranteed raise, no risk. Just make sure you're not away from your console too long because the stock will rise ever 5-9 seconds. When it peaks, sell the shares. I did the 3rd mission and then went to work, so instead of getting 100% of my money back + my investment, I only got 23% back.