I think you are talking about RV parks rather than campgrounds.
I've been paying $5 to $20 a night for water-electric RV sites in city/county campgrounds, $18-45 for full hookup sites in full-service commercial RV parks, mostly in the middle of the country. $30+ is pretty typical in season, it can get lower off-season.
It is not unusual to pay more at destination parks, in season at tourist destinations or urban locations. $40-60 at Nashville, $50-100+ at or near theme parks. On the other hand, you might get a space at a casino for $15 or less, it might even be free.