This cheat is very easy and you will have a lot of fun doing it.
First go driving around Chinatown and find an armored car.
If you already have one great, once you have the car go into any one of your safe houses and store the car, then save the game.
Second got to Friendly Fire and buy a crap load of pipe bombs.
After that go to your safe house, take the car out into the nearest street and unleash your pipe bombs on the car.
When it blows up there should be about $2,500-$6,000 - pick them all up then go back to your safe house and buy the car out again for $500.
Repeat the steps until you have enough money to last you.