1. AEGEE-ZARAGOZA can help you to find a room or flat shared with spanish people with option of cleaning included. The price is around 300€, depending of each flat, situation and conditions (internet, heating, etc). This is the best way to learn spanish and discovering our culture. You can see below the list of available flats by their street name. You only have to write to the email firstname.lastname@example.org telling that you are an Erasmus Student and asking for rooms. Good luck!
2. Also you can go to the HOSTEL «La Posada del Comendador», one night it’s around 18€ and you can look for flats directly in Zaragoza, we think in 2 or 3 days you will find a room in a flat. http://www.alberguezaragoza.com
Because in the UNI there are a lot of people that announce flats for rent.
3. Finally you can look for a flat in the next webpage: http://www.zaragoza.es/ciudad/sectores/jovenes/cipaj/cont/buscaranuncios.htm
There, you have to click on «alojamientos (oferta)»