It's a fee box & still first come first serve camping. several unofficial overflow sites too. It's certainly worth it for the well-maintained restrooms (then again, my visit was just before Labor Day weekend).
alternative camping: lower Big Lost River campground, south of Mackay reservoir. No fee, no drinking water provided (but river is adjacent). Pit toilets are there, clean except for some vandalism. Administered by Idaho Fish & Game Dept. The river is a refreshing place to swim after your hike. When we visited, we were the only tenters there & camground filled in early afternoon. Unfortunately, many RVrs had generators running much of the time (not against the posted rules strictly speaking).