I'm sitting on over 3M cr. and really, at least for me, its about buying and using cars smartly over anything else. When I first start playing through the game, I either use all of the cars I win, or I buy a car that will be able to serve multiple purposes. So, for example, my Lotus Evora was able to compete in all of the C-class races, plus the sub-300 BHP one, the MR race, the modern British class, etc... It saves money and its a helluva car.
Otherwise, racing online is a decent way to make some cash, that is, if you're good enough to be competitive. Its all relative really, and because I like to race, that's where I got most of my money.