I stayed at Centralia College last year and that worked pretty well for me. I know you can camp there for free but I chose to pay the $30 to sleep in the gym which also included towels and showers. I think you can also pay $35 at the door without a reservation. I have a bed that is only slightly softer than cinderblocks, so I was actually quite comfortable sleeping on the floor. Your experience might be different, though...

Oh, and be sure to bring ear plugs if you choose this option. people can make astounding array of whistles, hoots and twitters when they sleep.