i guess the cheapest hotel in amsterdam would be the City Hostel Stadsdoelen. there are sleeping rooms with 8 to 20 beds, so you sleep with other people in the same room. it costs per person 20 euro. i´m not really sure, but i think this hotel closes at 1:00 am, which would be really stupid. i wish it would be warmer in april, that camping outside would be possible...