Sorry if this is a repeat, but the easiest money maker BY FAR in my opinion is the 500pp World Classic Car Series using the Lamborghini Miura. The payouts range from $264k to $300k * 200% login bonus. I'm a novice compared to most people on here, and I win gold every time by 15-60 seconds in all 5 races. I believe I got the tune from The Tuning God Mr. Praiano. I'd be happy to post the tune if anyone needs it. Just send me a private message and I will post it.
My other easy money maker is the last race in the 710pp Dream Car Championship (Eifel) using the Toyota 88C-V Race Car '89. It pays $490k * 200%. Again, happy to post tune or share car upon request.
Enjoy the $$$ :-)