Washington State Parks charge $6.50 processing and $5.00 out of state per reservation. If you only stay one night at each park then yeah, it's $11.50 a night, but if you stay a week it's only a couple of bucks a night. The camping fee also includes the Discovery Pass fee, so you can visit other parks no matter where you are camped. Set up camp in a central location and visit other parks on day trips.
And yes,to get a decent site on the coast during summer best to book no later than early January. Safe travels.