Gatliff hostels are £14, or £9 to camp outside but use the hostel facilities (shower, toilet, cooking, sitting around in the lounge hiding from the midge).
There are a number of official campsites with showers etc, some of which have a communal building as lounge/kitchen. The ones I used in June were at Borve (Barra), south end of North Uist (shortly after getting off the causeway) and Lickisto Blackhouse (East Harris).
There are some semi-official camping areas (
http://www.westharristrust.org/camping/ ) and a few traditional wild camping spots that have nearby toilets, such as Huisinis or the beach just south of Timsgarry (£2 if you can find someone to pay).
On the return, I've found that the Tarbert - Uig ferry doesn't leave me with time/enthusiasm for getting further than Sligachan. There's the hostel and campsites at Broadford, but after that there's only official camping at Arisaig, Resipole (2.5 miles east of Salen), or Kilchoan (a mile and a bit past the Mull ferry turning), and there's a bunkhouse at the Glenuig Inn.
On Mull, there's a campsite a mile out of Tobermory toward Dervaig, unofficial camping at Calgary by the beach, a (very) basic campsite by the beach at Killiechronan, bigger campsites near the Lochaline and Craignure ferries, and a site at Fidden near Iona. I'm informed that the community centre by the Tobermory ferryport has public showers, but I've not looked.
Mostly, though, wild camping means no available facilities at all. Take a trowel for digging a toilet (deep enough, and clear of watercourses), and a water filter for filtering the local stream or loch.