Campground rates vary widely with the region, the season, and amenities. I'd say the range is $10-$90, with the heavy concentration around $25-$38 for a campsite with "hook-ups". Tent or "primitive" sites are often much less, but not always if in a high demand area.
Hook-ups are typically electric and water and will include sewer in many places. Electric may be 20A, 30A or 50A. You specify what you want and get a price quote. Nicer (and higher priced) places may add telephone, internet wifi and cable tv to the "hook-ups".
County, state and federal parks are less expensive than private parks, typically running $10 or so less per night than private enterprise for similar amenities in a given area.
If you rent longer term, e.g. monthly or seasonally, substantial discounts are usually available. 50% is not uncommon.