I just got back from Thunderhill (awesome track, but I admit it was my first track experience).
While I was there, Lance Keigwin told me that the Sportscar Club of America (owners of Thunderhill) keep the track rental rate low for small groups (with the aim to allow small clubs access to the track).
I have not talked to the track, but Lance's back of the envelope calculation was about $250 US per person, with 20 people or so needed (5K). I see there are some available dates, and I am wondering what the level of interest would be for this.
Ideally scheduled just before another track day with some other provider (to be clear: I am not trying to become a trackday provider. I am trying to organize a group trackday, should the idea not be revealed as too stupid or crazy).