To easily gain money on any racing level in A-Spec mode, find a race with a championship circuit and get a fast car. If you are low level and have the concept car or any car above the hp shown in the requirements or a little less hp as long as you can drive, then you can perform this trick. Select "Championship" and race the first two races then exit. Repeat as desired. The higher level racing, the more money you will receive.
To easily gain money, reach level 11 in B-Spec, and then buy a Lupo Cup Car with a lot of horsepower. Then, have any B-Spec driver enter the Lupo GTI Cup. Because of the horsepower, your driver will easily win the race in twenty minutes. Note: You can set your alarm to 20 minutes, take a break, then return and do it again.