Man I suck at this game man not making nearly as much $$$ as some of you guys. The Lester mission where you kill the jurors for the tobacco deal, and you invest in Debonair... I only got 80% return on investment. Sounds like a lot but its like ok so now I have 2 million with Michael. How the heck are some of you guys making what sounds like 50-100 million and even more per Lester mission?
Yeah, you have to save all the assassination missions (except for the first, which is required for the story) for after you've completed the game. The final score nets you north of $25 million, and you can turn that into $1 billion very easily with the Lester missions. But don't get greedy. If a website says you can get around 48% ROI on a stock tip, don't push your luck and keep sleeping hoping for a bump; eventually the stock dumps back down to basically a zero ROI and you'll have to scrap your save and reload from before to get the same returns. It's frustrating and stupid. But whatever.
*SPOILERS FOLLOW*
Let's assume you came out of the last mission with $30 million to invest (per character). Here's your breakdown (remember to always save immediately prior to investing all your money and starting the Lester missions):
Multi-target assassination: Invest all (literally, every single dollar available) in Debonaire (DEB) before you start. Once you finish, let the stock hit ~80% and sell. Take the money you earned and invest ALL (again, literally every last dollar you have) in Redwood (RWC). After a day or two it will rebound to ~300% of your investment. This is your biggest ROI, so don't skip it. With this mission alone, you should be up to around $162 million.
The Vice Assassination: Invest all your money in Fruit (FRT) before the mission starts. After the mission, let Fruit hit 45% or so and sell. Save the game, then invest all your money in Facade (FAC). Facade should rebound after a couple days to the tune of ~30%. This should push you north of $300 million.
The Bus Assassination: Don't invest to begin with. Do the mission, and after it's completed invest all your money in Vapid (VAP). After a day or two, Vapid will rebound for a 100% ROI. Now you should be above $600 million.
The Construction Assassination: Invest all your money in Gold Coast (GCD) before starting. After the mission, wait for your returns to hit ~80% and sell. This will bring you above $1 billion.
Do not get greedy. I did on one, and I had to manually delete a save file to see above a 10% ROI on subsequent replays. And I did the multi-target one back when I only had about $100,000 to invest (and I completely skipped the second step which is far more lucrative), so even though it seemed like a lot at the time, after the endgame it was basically worthless (your properties will earn you more than that in a week). If you want to be crazy rich, this is the only legitimate option.