The rare times you get a chance to make a large drug run, save the game first just in case it does work. These large deals will usually cost you about 250 thousand dollars but when you distribute it eventually you will make a large profit. If you save before you go on the mission you can still request it after you turn off the console, if you screw it up. Becareful when you make these deals, they are worth the trouble. Remember to keep the repairboat and repair vehicle cheat codes ready. Your profit won't be as big if you have to keep repairing your stuff.