I've recently finished all 69 missions in the main campaign, and everyone has about $26 million in their bank accounts, but that isn't nearly enough to buy all the different properties in the game (the golf course is $150,000,000 by itself!).
Apparently it's possible to become a billionaire, but most people do it through judicious use of Lester's assassination missions. Unfortunately a lot of people will be like me; finishing the game before looking at any tips, tricks or guides. So how can I now take each character's final money and turn it into more?
I've read theories about manipulating the LCN exchange (the market tied to your single player game) by damaging rival companies so their competitor's stock rises (or the target's stock falls).
For example, CoolBeans vs BeanMachine or Burgershot vs Up-An-Atom. The theory goes that if you blow up a bunch of FlyUS airplanes, AirEmu's stock will rise (or at least, FlyUS's stock will plummet). But as far as I can tell, these are only rumours and nobody has actually been able to make any money doing it.
There's also obviously the BAWSAQ, which is tied to fluctuations in both GTAV and GTA Online across everyone's game -- but this market seems to be far more unpredictable (as you might expect).
Note:The "invest in Debonaire" answer below does NOT WORK.
So how can I turn my characters in GTAV (not GTA Online) into billionaires?