I've ridden Powers....it's a small arena cross style track. If you're going that way....IMO....Big Air is a better choice. Less congested, bigger track, more friendly obsticals.
Portland is just West of Lansing, about 20 minutes East of Ionia. Great facility. I believe that you have to become a club member in order to do any open practice days.
Battle Creek is closer to you. Similar in terrain/obsticals (and membership) as Portland. Might be a cool option to join one of these clubs. Great guys, similar interests, etc.
Have you looked into M40, near Marcellus, MI??? I heard that they had some open practice there too. They have an oval dirt track for car racing and a MX track on the NE corner of the property. It's small, and I don't believe that they charge much. If you google it, you should be able to find a contact. I haven't been down there in quite some time, so I'm uncertain of the track layout now.