To easily earn money, go to the Driving Park Test course (large oval track), and set the laps to two or more. Use your best top speed car, and have a second player choose a slow car. Set the race to AI for both players and continue. Eventually you will get the jackpot race multiplier to "6". If it lands on you and you are in first place, you will win all of player two's money, and it will be added to your money and then get multiplied by six.