http://gtaforums.com/topic/603606-make-over-2-billion-per-character-easy-mild-spoilers/

Although this thread never went anywhere apparently, I'd like to confirm that this method does work (thanks for sharing, by the way).

Does anyone know how much should I invest in the company's... I invested 17 million with each character in the Augury Insurance Company after the final heist. I didn't save the game yet... So I'll get probably more but it should take a while, right?

Edited October 14, 201312 yr by Chr1st1an_UM

Just to let you guys know, I invested 25 million dollars in Hawk and Little (BAWSAQ) three days ago; I've just sold everything and got 72+ billion!

 

So yes, it's still possible to make a lot of money after completing the Assassination missions.

Edited October 15, 201312 yr by Sgt. Jackson

Well damn, if I would've known this I wouldn't have finished the game... I suppose I can create another save and start over, since I am at 85% complete and working on getting to 100% completion. But at least $2,000,000,000. I like that! :D

What would you do with 2 billion dollars? 200 million dollars is more than enough money in my opinion.