To go further with the Stock Buying/Lester missions: With Franklin, you can buy right away, but it won't let you switch to any other character while in-mission, so fail the mission on purpose and exit it. Then, switch to the other 2, put all their money into the stock that's about to go up, then go back on Franklin and do the mission. Get that money.
You gotta be careful though, cause them shyts WILL go down. Like with the 2nd assassination, I completed it, and was letting the shyts sit, while looking for the first bounty mission with Trevor (I STILL can't find that dude
). I died a couple times by Mountain Lions, and once when I stumbled onto some base in them mountains. When I got around to checking the stocks, that shyt had dropped damn near 100 points
Luckily I still had a save made shortly before at the high price. Loaded it up and cashed it out.