That's a cool model there Tony...and seems pretty fair. If my SCSU math is correct then..using BIR as an example---$80 entry fee (90 cars)should pay $1600 Win/$800 RU.
Overhead is going to vary WIDELY by track. That isn't something the racers really need to concern themselves with.
If a track has too much overhead and is trying to cover it with bracket race profits, they probably will not provide a program that is attractive when compared to that of a leaner/more efficient track.