Unless you get a nice owner who gives you a discount, camping will cost you in PA, NJ & NY. Places along the route south of Philadelphia will likely run at least in the high twenties/night and maybe even 30s. I doubt you will find anything free like city parks. Obviously, there is no established camping close to Philadelphia and I don't think it would be wise to stealth camp in the wooded areas of the city's public park system. For out of state residents, NJ state parks like Worthington near Delaware Water Gap, PA, are $25/night for non-residents. Once you get into NY things probably won't get much cheaper. Commercial campgrounds were pretty expensive when I last looked into them a few years ago. Some of them were in the mid to upper 30s. Out of curiosity, I priced a tent site with no utilities for next week at the KOA a little above Port Jervis. $36. Probably more in season. There are one or two state parks near/in Hyde Park. Don't know what they charge.
I rode a short portion of the route (Port Jervis, NY to Philadelphia) back in May. Drove a one-way rental to the start and camped a few miles outside of the center of town at this place:http://www.rockviewvalley.com/rates.htm
$30 for a site with no utilities. The owner only charged me $15. He said it was because I was on a bike, but I think maybe it was because the place really was just gearing up for the season. The pool wasn't even open yet. The next night I stayed at Worthington for $25. The third night I stayed at Dog Wood Haven just off route in PA across from Milford, NJ. I was again charged $15 since I was on a bike. (I was also offered a free beer by the owner.) They don't have a web site so I don't know what the normal rate would have been.
I am hoping to ride part of the route next June to get to my 30th high school reunion in western MA. Guess I will have to suck it up and pay the price.