Depends on where you are and sometimes the degree of "improvement" of campsite. Typically $14-18 for RV sites in state parks in Oklahoma (most have no entrance fees). I've paid $20 to $25 in Texas (plus $2 per head entrance fee), $25 for full hookups in Michigan (plus per vehicle entrance fees), $12 to $30 in Illinois (no entrance fees but rates go up for holiday weekends).
I think you might encounter higher fees for public parks on the West Coast, particularly at beaches. If you regularly use state parks that have entrance fees, an annual pass is usually worth the cost. It certainly paid for itself when we were visiting the Texas parks while my daughter lived there.
Where I travel, I more often use municipal or county park campgrounds, and USACE recreational access areas (which are usually half-fee for campsites with my Senior Pass so $16 a day drops to $8).