For cheep accommodation in Zagreb there are here four possibilities offered. Please feel free to add or suggest others. For reservation contact them directly:

Hostel Ravnice
30 beds (2+4+12 rooms) / 15€ per person / new hostel!
http://www.ravnice-youth-hostel.hr/

Hostel Fulir
right in the center / 14 beds / ~ 19 € per person / opened this spring!
http://www.fulir-hostel.com/

Omladinski Hostel
right in the center / 210 beds / for diehards
prices ranging from 6 bedded rooms/per bed 10.65€ to single room with shower & toilet 31.85€ + sojourn tax per person / overnight 1,00 euro
http://www.hfhs.hr/front/index.php?r=154498892&topic=hostel&sethostel=zagreb&jezik=en

Zagreb hostel overview:
http://www.hostelz.com/hostels/Croatia/Zagreb

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Hotel FALA
prices from 36 €
http://www.hostelz.com/hotel/922-Hotel-Fala