Occasionally you find a place where you can stay free (ACA maps tend to indicate them).
Depending on where you will be you can probably stay for free more than occasionally. On the TA we stayed free about half the time and quite often paid less than $10 when we did pay (over a 73 day period). Last Spring I never paid for a site on my 10 day tour, but did stay in $40 motel rooms several times.
Most small town parks are fine. In the west if there are no signs that say it is prohibited I usually set up near the picnic pavilion. Asking around in small towns will usually result is a spot to camp. If there are police I ask them. If there is a store clerk available, I ask if they think I will be run off (rather than actually asking permission). I try to set up early so if I am going to get kicked out it will be early enough to find another spot. I have not been run off yet.
It may be harder in the east, but in much of the west it is usually pretty easy to find a free spot to pitch a tent for the night. At least that has been the case for me so far.