10 Grand Every Ten Seconds:

For those who don't like to use cheats and earn trophies/achievements: Go to the airport. Grab a plane or heli. Fly it in between the tall electrical poles around the airport. If done correctly you will see barnstorming come up and if using ps3 click and hold R2 for an additional bonus. Tip: there is a crashed airplane in the water next to the airport runway. Use those poles for maximum gain.