In order to save some money on this trip Matches and I have been camping for free wherever we can.
There's a cool website we found that allows you to search for campsites in the area, and it will show you any free or paid camping areas nearby – including some free campgrounds in National Forests, paid campgrounds in National Parks, Walmart and Cabela's parking lots, and places you can just pull off the road and sleep.
This led us to a rest area in Pennsylvania, a mud road in the desert of New Mexico, Kaibab National Forest – a mile from the Grand Canyon, a pine and Sequoia forest in Sequoia National Forest (not the National Park), three miles from a hazardous waste facility with fields full of pronghorn and an abandoned marble mine in Utah, and a gravel parking lot surrounded by cornfields in Illinois. All for free.
The downsides to free camping are obviously things like no bathrooms, showers, or potable water.* It's definitely worth it for most of these sites.
While we did stay in a hotel/motel about every other day we saved a good chunk of money, and had some cool adventures by camping in these places!