Amsterdam is a tough place to find cheap, non-squalid accommodations. You might want to slightly raise your max price per person and rent out a vacation flat and then save money by cooking your own food while you're there.
You can find vacation rental flats through:
http://www.vrbo.com/vacation-rentals/europe/netherlands
Also, look on tripadvisor.com--there's a lot of suggestions on their forums and reviews of flats, hostels and hotels.
Good luck!