Kalama Marina is pretty cheap. You could also just sleep in the parking lot. Right across the river is an island with a piling field on the downstream end. You'd be safe anchoring there. Even if the incoming tide is big, at worst your current will diminish to about 1/2 knot. This time of year there should be plenty of flow. Lower down from there, up to somewhere above Cathlamet, the current will in fact reverse.

There is a mobile home park at the south end of the Kalama parking lot, and a campground back north one exit on the freeway. I use the campground a fair bit. This time of year, I use the motel, however.