Depends on your rig and what you are willing to put up with. If you have a Great Big Giant Class A and expect full hookups, $30 a night is pretty cheap. If you are in a converted van and will be happy with no hookups, there are plenty of options for free or almost free.
If you are at least 62, the recommended Maryland Golden Age Pass is a great idea. Ditto the Federal Interagency Lifetime Senior pass, also $10 and good for free day use many places (half-price others) and half-price camping is another good idea. The latter has gotten me lovely sites all over the country for as little as $8 or $9 a night (with electricity).
There are also some Wally Worlds, Cabelas, Cracker Barrels and other big box stores where you can stay for a single night for free, no hookups, of course.
You might also find a casino, especially an Indian casino, with a cheap campground. Although maybe not in Maryland. Casinos do this as another way to attract gamblers, but of course, they can't force you to actually gamble. A couple of the most beautiful and cleanest campgrounds I've ever stayed in, with hookups I might add, have been casino campgrounds. $20 a night, full hookup, grassy site, sparkling clean bath-house. Can't be beat.