Easy money:
Save up for a car and buy it. It helps if you have a high license. The Mitsubishi GTO (SA class) is recommended. Upgrade it fully, and then choose "Race", "Special 1", and either the 400 or 1000. Select "Set up" before you race and make everything low. Race and win the cup. If you cannot win the cup, then upgrade everything one point until you can. That should earn you at least $147,000. The next time you race the cup, upgrade one thing on your car by one point. This will allow you to defeat your ghost (which rides as the last car after you win). Keep doing this to get a lot of money in no time. Note: Make sure you only choose set up the first race and not at all throughout the tournament or else it will not work.

The following trick will cost about $60,000 and will earn more than $500,000 and more cars and bodies. Go to Carrozzeia. Choose an A class (2000cc), use a In-Line 6, and choose NA or the others. If you have more than $100,000, choose Mid-Engine 4WD or Rear Wheel Drive. Choose Razza. Upgrade everything (most importantly clutch, engine, muffler, and tires). Go to the 400m Dragway and set up for everything at the lowest. You should win easily and turn up one level every time (to defeat your ghost).

Go to championship mode, then select the "Buy A Car" option. Go to the "Used Cars" at the bottom. Go to the next screen and buy the blue Mazda Miata for $9,000. Go to the "Race" option, select "Open Race", then choose the McDonalds race. Win it with the Mazda and you should receive a lot of money and another Mazda. The other Mazda is not as good, can be kept as a spare (or you can upgrade it). Repeat the race as many times desired to keep earning money and new cars.