Double ditto on Corps of Engineers campgrounds, state parks, and some city campgrounds. Prices will vary widely based upon proximity to major metro areas with heavy tourist traffic e.g. Washington DC; New York, etc.
You can use Allstays.com to find campgrounds that are marked more within your budget
http://www.allstays.com/Campgrounds/They also have a comprehensive link to all the state parks in the USA
http://www.allstays.com/Campgrounds/state-park-campgrounds.htmCorps of Engineers campgrounds can range from boondocking to full hook up dependent upon where you are
http://www.recreation.gov/unifSearch.doAnother option if budget is constrained is to "part-time". Just travel for part of the year as we do.