On my Northern Tier experience (East to West), which I followed as far as Muscatine, Iowa, I only paid to camp before I started in Bar Harbor, Maine, when I first hit Lake Ontario in upstate NY, and on the border with Ohio. I think that's it. The rest of the time, I camped in town parks, commado camped, used Warmshowers.org, or was taken in by very friendly locals. To keep things really low budget, you have to go beyond a conventional comfort level and seek out unusual camping options. Sometimes these produce the best camping experiences, however. For example, I rolled into Conway, NH, did laundry, got food, etc., but found the camping/hostel options were too high for my budget. I looked at a map and found that I could get into National Forest land not far outside of town. Heavily loaded with food for a layover day, I pedaled into the beginning of the White Mountains and, after a little anxious riding in the fading light, found a perfect camp next to a pond with a grand view. The spot remains one of my fondest memories of that long and very interesting ride. Had a stuck to conventional options, I never would have camped there.
Be adventurous--you won't regret it (usually!)