The vintage races could bring in up to 10K people at $25 a head, Maybe even a few more. But that does not compare to the revenue generated by the entry fee.
No guts, no glory.
Last I heard there were over 1000 requests to attend the Indy event. There's 400 cars willing to pay what ever the entry fee is that could run in some class of SCCA competition. The $800 price is partially due to the 4 day nature of the event. That's only $200 a day and from what I can tell each group get's a fair amount of track time.
I think this CLEARLY shows that with even a little marketing filling the joint should not be a problem.
And you can't compare this to COTA. I understand that the club didn't do as well as expected on that one, but how can you compare a brand new track to the most historic site in American racing?