You have a good chance of hitting snow on the icefield parkway in May. The national park campgrounds will be closed - which essentially means free -just don't expect any facilities and stay out of sight of parks employees.

Outside of the long weekend, it really isn't tourist season yet, so you might find some cheap motels. I don't know how many hostels are on your route but there are anumber on the icefields parkway. Some of them may not be open in May - best to check.

http://www.hihostels.ca/hostels/Alberta/index.html

The upside of course, is that the roads should be relatively quiet, the days are long and the migratory birds are returning. Should be a great trip!