There are many FS campgrounds along Hwy 75, both on the Ketchum side along the Wood River, and on the lakes on the Stanley side. Don't know the fees because I don't use them. Here's at least a partial list:
http://www.fs.fed.us/r4/sawtooth/recrea ... ngsnra.htm
There is also a lot of "dispersed camping" but not right on the highway: you have to drive up a side road.
If you don't already own a copy, buy "Idaho, A Climbing Guide" by Tom Lopez (shameless plug- Tom is a friend). It lists almost all major Idaho peaks, along with access to same. Those access roads usually take one to good camp spots.