When moto traveling, I refuse to pay for camping if I can help it. The only time I 've paid was because I was with friends who wanted to stay at a 'real campground', but even then, we once camped-and-dashed at a campground on the Blue Ridge Pkwy because the experience was like sleeping on the set of the Jerry Springer show being filmed live in a central Florida trailer park. We refused to be taken for $13 for that experience and hauled ass before the ranger did his morning rounds.
For wilderness camping, free comes easy, but it's a little more difficult when near civilization.
Here's what I do:
1) freeload off of contacts from the Net. I don't hessitate to contact a firend from a forum to let them know I'm passing through and ask if I can camp or grab some couch. I would offer space for them and I expect them to want the same from me.
2) Sleep in rest areas. I've slept face down on the grass at rest stops, as well as sleeping for 5 hours on a picnic table in a pavillion with my bike parked next to me.
3) Truck stops are great and truckers won't mess with you. I've stept face down on asphalt between 2 tractor trailers using my tent as a mat. I've also camped inconspicuously on the grass strip at the back of a truck stop on many occasions. The nice thing about truck stops is that you can also get a shower there.
4) Other places I've grabbed at least 5 hours of sleep include: the shoulder of the road at a scenic overlook, ferries, gas station parking lot (with permission), in the woods off the highway along a powerline.
PS: email me for the GPS coords of an awesome free campground in northern Maine.