People's definitions of "cheap" are different.
At zero, there is "couch-surfing". Around 30 Euro will net you a bed in one of AMS's excellent youth hostels. Hotwire has a 4* at $88 for one date next April. You could bid on Priceline at the different star levels. And I've heard people describe under $200 as "cheap".
A little more specifics?
Romelle