Okay so what are the exact settings then?
Custom Race any track, pick N100 or maybe N200 car. I'm trying it on Red Bull Ring right now.
Race Type - Endurance
Time Limit - 25 minutes
Starting Grid - 20
Fuel Depletion - 10x
Boost - Off
Opponent Category - Gr.1
Opponent Difficulty - Professional
Tune your car however you want. Race. You'll get lapped once within the first 5 minutes, so try not to get wiped out. And then when the CPU tries to pit, it can't fill up on fuel so the cars will start dropping off around the 10 minute mark, and you should get 1st by around 21-22 minutes.
I'm still testing it out so things may change. This will probably get patched out around next week.
EDIT: There's no handicap bonus. You do get about 300k, maybe 450k with Clean Race Bonus. If you're willing to increase the Time Limit, you'll get more prize money.