I think that the "simple math" comment was directed at the error in the calculations. 90 x 200 = 18,000 for 2 classes = 36,000. But many local races offer 200% or 300% Pro purse meaning the track will match the entry fees by doubling or tripling. If you have 18,000 per class from the riders, and the promoter gives 300% payout, you are looking at splitting 44,000 for each class. That gives enough to pay a decent amount back through 40th place for making the show. Say $500 for 40th and on up little by little from there......
No matter how you look at it, the riders are getting screwed.
And FMX is different sure, but the concept is the same. Riders make the event that people are paying to see, they shouldn't have to pay to get in, and should be paid like any other sport.