Easy Money $270,000 per hour (Not anymore)
EDIT: THIS NO LONGER WORKS
If you need help making money in GTA Online watch this video! This is a speed run of the Gerald mission called "Violent Duct" which pays out $9,000 for less than a minute's work. Each run like this this took us an average of 2 minutes (including load time) so doing the math it equates to $270,000 per hour. As of today (Oct 7th, 2013) it is still the fastest way to make money even at rank 40s & 50. Stay tuned to the end of the video to see the rank 50 Merryweather Airstrike ability.
I made this video to illustrate one of my main problems with this game. This is easy money but it's a symptom of a serious flaw in progression when a lvl 50 would rather grind this noob mission instead of doing high level missions because this pays better.