Running taxi missions can get you a small nest egg, which you then want to invest in the horse races - the odds of winning are pretty unrealistic. I seem to remember it being the same odds of success regardless of the odds on the horse.
One pretty easy way is to finish all the challenges in flight school. Once that is done, a Hunter (modeled after the real life Boeing Apache Helicopter) will spawn at your air strip. When you attempt the vigilante missions you'll get $100,000 once you reach level 50 and the amount goes up for every level.
The vigilante missions mainly involves the destruction of three slow moving cars with stupid gangsters inside whilst the helicopter is armed to the teeth with a self-targeting machine gun and two unguided missiles and it is extremely durable.
It is really easy once you get used to the controls and I made $100,000,000 [$100M] in just 40 MINUTES!
Try it and see for yourself.
