I'm in Fond du Lac, and there are two tracks here. One is right along Hwy 41 and only costs $25 for a one year membership. The other is south of Fond du Lac. I haven't been there yet, but that is where all the racing goes on.
I am a member at Valley Motosports (Just south of Fondulac). It's a great track but it costs $200 per year. They have a Friday night series that is very laid back and fun. Look in Cycle USA for the schedule. It looks like Aztalan near Lake Mills is your closest track. I think that track is better than Denmark and Valley. If I lived near it, I would have a membership for sure!!