We live here. Now I'm not a fan of state parks for various reasons. The rates vary by what state parks you visit. You can definitely get under 33.00 a night. Some even have full hookup. As far as not being full - I don't know where you stayed, but at that price it was probably one of the more popular parks, and in that case I know for a fact that pretty much starting in June you " will not" get a reservation much less just driving in as they are usually booked until the first of September. Anything before June there are probably plenty of spots available. Anything after you got lucky because someone cancelled.
Michigan has a 100% reservation system and campers start reserving January 1st with a 6 month window.So by March 30 the seasonis booked in our state.
Most nice full service campgrounds here are more than the $33 you compare to our state parks. Michigan has some really beautiful state park campgrounds located on our inland lakes and all along the Michigan shoreline.On the lakes they're more money. I don't complain about prices at campgrounds when traveling as there are plenty to choose from that would fit my budget.
The Internet is really great for planning trips ahead of time as all prices are available to review, no surprises.