I'd say anywhere from, say $65 to $100, depending on the costs associated with putting on the rides (vets, potties, meals, awards, campsite, permits, etc.).
But I'm in the NE region so I could be off a bit on that.
And hey, when you join, be sure to look up the mentors in your region/area. If you click on the tab for "Education" you'll see "Regional Mentors" listed there -- if you can't find someone, drop me an email at firstname.lastname@example.org and I will do my best to get you hooked up with someone if you're interested in having a mentor help you along as you get started.