I'll second the above reommendation for loquo. 900 Euros seems like a LOT of money. I live in Moncloa (just north of the center) and I pay 380 Euros per month(including utilities). I live in a piso with my roommate from the USA and one other woman. Especially for a short time, it might be hard to find an actual apartment to rent...a lot of places have a one-year minimum to rent an entire apartment. Also, try looking around the university campuses or notice boards...people will post signs when they are renting out pisos or habitaciones.
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La vida es como la espuma...por eso hay que darse como el mar.