There are various ways to make money. Do some good paint jobs and sell for 50,000 or put them on a car and put the car on the auction house.
Play the auction house by buying low and selling high. Selling high is easier if you buy a plain car and stick a good paintjob you hsve done on it and sell the car on the auction house.
I make my money doing rivals events. Set a slow time to start with and then pick a rival just in front of you. Keep doing this and eventually you will get to 100,000 credits or more per rival you beat. you also get affinity along the way.
Also a lot of people are close to a few hundred thousand and they don't know it. Go to your profile, affinity and find any manufacturer's where you are in the last quarter of the guage. Take those cars out for a race to "cash in" the next affinity level.
My head hurts.