My friend and I went to Amsterdam for three weeks in the summer in the late 1990's and she and I rented a room at the dorms for the University of Amsterdam. It was a very common thing to do, and it was cheap. I think it was something like $70 USD a night, so $35 for each of us and there was a shower. The beds were simply mattresses on the floor and of course the rooms were extremely narrow like all of Amsterdam, but I was still glad to have fairly decent accommodations when we compared stories to other travelers we met. Hope you have a great time.
