Fastest way to get rich in Grand Theft Auto 4 - Episodes of Liberty city: The ballad of Gay Tony Singleplayer

Betting on and spectating cage matches is indeed the fastest way to make money. Similar to the OTB horse race trick in San Andreas, the best strategy is to save at the nearby safehouse before you place your bet, and then load the game if you lose.

Since you essentially can't lose, betting all of your money is the way to go. If you've got a small nest egg of a few thousand, you can very quickly grow it into a few million with this method.

Drug wars are also good. I have 13,00 bucks and got RPG, auto sniper rifle, ak47, p90, tons of grenades, auto shotgun, And desert eagle pistol