Go to an ATM, then cause a traffic jam to the area. Wait for someone to take money from the ATM, then kill him or her. Make sure to block any nearby roads so the ambulance cannot get to the murdered victim. Take the money they dropped on the ground. Then, run a short distance away, and go back. The money should have respawned on the ground. You can repeat this as many times as desired. You can also kill more people taking their money from the ATM to increase the amount of money that respawns.