Yes, there are plenty of low cost campgrounds to stay in. State parks and federal camping areas (i.e. National Forest & BLM, primarily, and most BLM areas are in the west) are typically less expensive than private places. We've stayed in several National Forest & BLM areas that are even free, but they like more of these federal lands, there are no hookups of any kind. And generally, these areas are NOT close to interstates. But these are the areas I usually strive for, since they are generally much more scenic and less crowded. An exception is National Parks -- they are usually a zoo.
I've found this book to be occasionally helpful (there is an east coast version, too):
http://www.amazon.com/gp/product/0937877492
We've traveled all around the country, and in our experience, sites can range from $0 to $100 (yes, at a campground), but I'd say the average is more like $20-$35/night, including water and electric. Many parks have electric only hookups, sometimes in addition to water & electric, and they cost less. Some campgrounds will let you dry camp (i.e. no hookups) on their property for a reduced rate....maybe $15, but they won't advertise it since most people don't do it. But there's no harm in asking.
Dave