Easy money
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.
Find an area with a lot of people. Then, position yourself a decent distance away, and enable the "Spawn Jetmax" code. The boat should spawn above the people, fall on them, and kill them without raising your wanted level. You can then pick up the money dropped by the people killed. Repeat this process as many times as desired.
After completing all of Stevie's 30 vehicle thefts, go to the garage in south Bohan. Then, enter one of the "Spawn" vehicle codes, and take the vehicle to the garage to get the indicated amount of money as long as the vehicle is in perfect condition. Repeat this as many times as desired.
Cognoscenti: $9,000
Comet: $6,000
NRG-900: $2,500
Sanchez: $1,200
SuperGT: $11,000
Turismo: $11,000
After completing all of Stevie's 30 vehicle thefts, take a taxi to the Perseus on Pyrite Street and Bismarck Avenue. Walk to the Grotti car dealership with a baseball bat. Use your baseball bat to break open the window on the east side of the building near the tree that has a Turismo parked behind it. Try not to damage this car. Take the Turismo, and drive it to Stevie's car shop. Taking the Charge Island bridge to Bohan is the best route. Stevie will give you approximately $10,000 for the car, depending on its condition. You can then take a taxi back to the dealership and do it again.
Steal an armored truck, and blow it up to get approximately $1,100 from the piles of money surrounding the wreckage.
There is a glitch in Aldeney at the intersection of Tudor and Port Tudor, close to the prison where you visit Gerald. You can find a total of nine armored trucks arriving back-to-back. Highjack one, then block the road. Another one will appear. Blow them up one-by-one. There is a total of approximately $1,100 on each truck, which is especially useful when just starting out or if you need money.
Steal a Securicar, and blow it up to get approximately $1,000 from the piles surrounding the wreckage. To do this without getting a wanted level, you must drive it and get it damaged until it catches fire and explodes. Do not attack it with a weapon or you will get a two star wanted level. If the smoking Securicar stops running and still is not on fire, use your cell phone to call anyone, then hang up while it is dialing. The Securicar will start again, allowing you to do the last amount of damage to get it to catch on fire and explode.
Go to a strip club, and kill the strippers. Each one drops about five or six wads of cash, but all the guards will attack you.