I would second the state parks idea. Most run $25-$30 a night. A couple bucks more if you want an electric site. I think Assateague is $40 for an electric site at the state park.
There are also several National Park campgrounds that are very reasonable. Catoctin Park north of Frederick, however the sites are on the smaller size (FYI, this is the park that Camp David resides in, when the President is in residence, some of the roads are closed, but you still have campground access). Assateague National Seashore, which is adjacent to the State Park is also very reasonable, however, it has sparse facilities....cold showers and (clean) pota-potties.
A couple of or favorite state parks besides Assateague (state or federal) would be Cunningham Falls State Park north of Frederick. It is adjacent to the Catoctin park noted above. Nice size campsites, some with electric, nice beachs and a 50? acre lake. This park is just 15-20 minutes south of Gettyburg and makes a nice alternative to the private parks in that area.
If you really want to boondock, try Green Ridge State Forest in western maryland. You can really get away from it all there, but you have to be judicious about you sites as some of the roads are not RV friendly (but many are).
Pennsylvania state parks are even a better deal, I think they are $15-20 a night. We visited Ohiopyle this summer and had a great time, what a beautiful area. Of course we visited Falling Waters while we were there, well worth the time and money. One thing I like about PA parks is that they do not allow alcohol. I drink myself, but I can do without. This policy seems to keep a lot of the yahoos out.