The only way you can get vast amounts of money quickly is by maximizing your investments before and after Lester's assassination missions, so some
spoilers regarding those missions in the following link:
https://docs.google.com/forms/d/1VLK0HCFWmfb7uG4_tzurGGBVY1WZBxF-2iKRwNojZRs/viewform
I'm on step 7 right now, the only thing this guide doesn't take into account is:
1. When the stock hits bottom (and what is the lowest value)
2. When the stock hits its peak (what is the highest value)
3. How long it takes to reach 1 or 2
4. How long the stock stays at its bottom or peak
Basically you will be utilizing the bed save with Franklin a lot to pass the time so the market moves faster (some stocks will take 1 or 2 saves to bottom out/max out - others will take 12+ to hit peak as an FYI):
I wrote down a few of them just so you can maximize your earnings (all of the other guides I've seen don't have these written in, some have maybe one or two expected results tops):
I think Debonair tops out at ~51% return at ~$1066 a share (or maybe I mixed it up with Redwood)
Redwood tops out at around 300% return (after it rebounds save one or two more times and it will explode to around 300%)
Facade maxes out at 33% return at $570 per share ($202 low point)
Vapid tops out at ~147% return at ~$500 per share.
Gold Coast tops out at ~82% return at ~$250 per share.
The thing to mention here is to have a save prior to accepting his mission WITH YOUR MONEY CASHED OUT!!!!!! and keep that save in case you mess up or sell too early/too late.
Because BAWSAQ, though affected by your offline decisions is still an online-tied/dependent market -- so if you miss the boat on a stock peak it will drop and keep dropping despite reloading a previous save post-mission, in this case you have to make a note of the peak price and reload the save before the mission and do it over and then sell at the peak you jotted down.
Also, if you played these missions already and didn't max out your investments with all characters you CANNOT replay them and play the stock market game, you would have to start a new game to take advantage of this banking strategy.