Go to an ATM and cause a traffic jam to that area. Wait for someone to withdraw 
money from the ATM, then kill that person. Block any nearby roads so that the 
ambulance reach your victim. Take the money they dropped on the ground then run
a short distance away. Return to the scene and the money should have respawned 
on the ground. You can repeat this as many times as desired. You can also kill 
more people and take their money from the ATM to increase the amount of money 
that respawns.