After you kill an enemy, press Circle on then. 50 Cents will kick the person, and most of time cash icons will appear. If you arer lucky, you can get lots of money by the end of the level
Do all the missions as far as you can. Then, quit only the missions to take the subway back to previous levels that you have completed to get more money to buy the cheats from Iceberm.
After completing the game, go to Iceberm. Buy "Invulnerability" and "Infinite Ammo" cheats. Then, go back to the subway entrance and choose the boat level. Repeatedly complete that level. This is the level where you must get the drugs, making you a lot of money. Use that money to buy more cheats, music, clothes, and different ammunition.