-To get money after killing an enemy, press Circle to make 50 Cent kick them; cash icons shoudl appear. Additionally, do all the missions as far as you can. Then, go to the subway entrance and choose a previous mission that you have completed to get more money in that mission.
-To easily gain money after beating the game, go to Iceberm and buy the "Invulnerability" and "Infinite Ammo" cheats. Then, go back to the subway entrance and choose the Boat mission. Repeatedly complete that mission. This is the mission where you must get the drugs, making you a lot of money.