First this is for those who have gotten to Class S.
You will need to get a fast car. Such cars include the Ferrari F430, Audi R8, Ford GT or the great handling Tuned Viper car.
Now race the 680p Daytona race which awards $32,500 Cash for winning a 5 lap race. This should be really easy with any of the fast cars mentioned above. There are penalties in this race so you may have to give it a few goes until you can run a clean race and take the money. But once you have mastered it, you should have no problems winning every time.