Another idea: If you're a registered adult leader with th Boy Scouts most Scout camps will let you pitch a tent for next to nothing and there are hundreds of them around the country. The same is probably true for most other youth organizations that have similar camps around the country (Girl Scouts, 4H, religious groups, etc.)
On a trip to KY/TN we called ahead and found a local Scout camp that let us pitch tents in an unused area while their big summer camp was in session. It would have been free but we opted to eat with the rest of their residents so we spent $5 per person. Great fried chicken dinner.
An update on this comment.
I'm in the process of planning a trip for our BS troop to Washington DC this summer. I was looking for places to stay on the way out; while out there and on the way back.
I found 2 Scout camps where we could stay enroute and on our return. We won't even have to pitch our own tents as they're letting our group of 20 stay in their pre-erected tents. Cost is $25 each for the whole group.
While in DC I found a Scout camp within 20 miles of the city. Cost for our group will be around $50/night...a steal in comparison to anything else that close.