Complete the Life Invader assasination and then invest all your money in the stock when it trades for $3, $4, or $5. Sell it anywhere above a 50 percent ROI. I bought some shares for around $3 and sold at $11. I made 500 million in one hour of doing this. It breaks the game a little bit but screw Rockstar for being so stingy with mission payouts.

Every character is close to a billionaire although I had to drain money buying all the properties. I have yet to complete all of the assasination missions except for the Betta Pharmaceuticals one. I only did that mission to get Franklin a mansion in the hills with a swimming pool.