Yeah they nerfed that some months ago. Before you could have it so that the GT1 cars run out of fuel halfway around 1 lap of the Nurburgring, letting you earn huge bonuses.^
About 300k or so without Clean Race Bonus. It's about half of BMB. A 40 minute Endurance race is 500k, which is better than the Endurance races in GT League. I thought you'd get a handicap bonus, but that seems to be reserved for GT League events.
Frustrating