Go to Bruski's Scrapyard and break into the random car located near the car crusher. Drive the car into the car crusher and crush it to get about $400. Enter the car you came in and drive it out of the scrap yard through the north exit. Stay on this road until you drive under a bridge. Then, turn around and return to the scrap yard. Another random car will have respawned near the car crusher. Repeat as desired. This will also unlock "Proper Scrapper" trophy.
Go to a store and spend some money, then pull out a gun and rob the store. You will get more money than you would by just robbing the store alone.