May I ask - did you pre-book the hostels or just turn up and find space? And what time of year are we talking here? I've heard good things about these hostels so they'd certainly be another accommodation option.
Yep, I pre-booked them. I went in the high-high season (July) and they ran full every night, at least the smaller wilderness hostels outside of the "big towns" (which are Jasper, Lake Louise, Banff townsite, and Canmore). You may luck on a spot, but I wouldn't count on it. (The "big town" hostels may be easier to find space on the fly, but they still can book up.) When I toured the Icefields Pkwy a few years back, I designed the tour here to hop from hostel to hostel. It's pretty easy, since the biggest distance between hostels is about 40 miles.
As for price, if you are travelling solo, I found that a dorm bed in one of the wilderness hostels was not much more than camping. Campgrounds looked like they were running around $22 a night per site and dorm beds start around $25 (plus fees.)
You might not be able to do it all by hostels, but you might be able to squeeze a few in. And the bigger hostels in the "big towns" have good services (laundry!) when you need to take care of that stuff.