In Washington state, it really depends on the CG and the quality of the site. They charge more for sites with a view, full H/U's or anything they consider "premium".
I would go to the reservation web site and look at CG's you wold like to visit and check out the prices. Expect any site with h/u's to be $20~$40+.
Also, if you are legally disabled or perhaps a veteran or disabled veteran , you can get at least half off (dis. Vet = free camping I think) after obtaining a Disability Pass (good for 5 years).
The good news is our state parks are better (IMHO) than private places. They are MUCH roomier and in beautiful settings.