I don't believe there's any admission cost. Tomorrow you'll be parking away from the field (only MVs, registered campers, and other people that need access to the rally field will be driving in, if I recall correctly) - out in front at the school building, and there will be a shuttle that will give you a ride down the ~1 mile access road to the rally field. If I remember correctly, they ask for a small donation for the ride.
There's a good turnout this year; a good number of military vehicles, and camping is full - and you can't beat the weather, 80 degrees and sunny.