When you get to the Champion level go to the map, filter your challenges so you only have speed challenges showing.
Then when you look at the top left of the map and there should be one that says 180 In Heavy Traffic and the gold reward is $70,000.
Make sure you use a fast car(Saleen maybe).
Once you start the race you should turn around and keep going straight. After you beat it press A and then press Y to restart it, and when you do it 15 times you will have around 1,000,000 dollars.
You should be able to do each challenge in around 10 seconds.