Any fast way to earn money?

I am trying to complete the campaign missions but i still drive the car i got from beginning. The Subaru One, but i can hardly make any progress, as i have literally no money (i didn*t even figured out how to earn it in a good amount) and all the cars are just to expensive. And this is why i can't beat the top 3 races. Everytime i complete a race i get 500$ but this grinding takes ages.
I thought of getting the Aventador for around 200k$ but so far i managed to get like 30.000$ and now i am kinda stuck now.

Did i miss something? The last races i completed i always was the last and only got 500$ as well.

Are you certain you're selecting new races everytime? As far as I remember, 500 was what you'd get for grinding out the same race over and over (which you absolutely don't need to do). The money rolls in fairly easily, and getting new vehicles is usually pretty easy as well.

Well no, i played all the campaign-related mission, (the ones where you would have been called for), but all the other free-races that have a star above their logo i did not started yet. I only once was able to get 4000$ but, it is really hard to get any money

Have you been following the walkthrough? You really shouldn't be having any kind of trouble with the early races, or with making money at any level. The game is pretty forthcoming with progress in that way. If you're not following the walkthrough, I think you should check out the one we have here on Trueachievements. It's pretty comprehensive.

Lol during the latter half of career I was getting 12K per race without fail. Had no problems with money, very easy to earn.