When you gold licenses, or clear the event levels ( novice, national, international, etc ) you get prize cars which are handy to use in future events so that you don't have to buy anything. If you're only doing the career mode, like @S_Bridge said, the Red Bull Championship pays a lot. The Sierra Time Rally has great payouts though its only once. The best money maker is to log on 5 days in a row and get the 200% credit bonuses. I got all the way to the end of International B with about 20 cars.