I think you can come on the day, buy a 'come and try' licence which is essentially a 1-day CAMS pass (about $20-ish), and umm.. i think you can just pay for a Mini Car Club membership for $20 or something just to make it all official. But my memory is failing me at my old age (of 21) so i'm not sure.
Basically you can walk away with change from a $50 once you've paid for memberships, the fuel you'd use and a couple drinks.
I'm not going; paintballing calls me and so does work in the early arvo