The Championship Racing in the Event List is really where you can earn a lot of credits. I believe Turn 10 put these in to replace the Endurance Races from Forza 3. Some of these races can be long and take considerable wear and tear on your vehicle if you have all the assists turned off with simulation steering and full damage turned on. These are the boxes on the far right side of the Event List. If you come in first in all the Championship Races you will earn a minimum of 3,743,400 credits. There is a qualifying race (Heat 1) and the Championship race (Heat 2). If you fail to qualify in the top 3 for the qualifying race you will have to run it again. The minimum credits earned doesn't take into consideration any Driver or Affinity Level progression or bonus credits you may earn in the races. For me just doing the F-Class Championship Races I earned well over 400,000 credits. So if you're thinking about cheating, because you don't have enough credits there really is no reason to. You can certainly earn enough credits racing from the Event List.