I've just booked HI Hostel Florida
The prices are very good, around 20 US$ per night on shared rooms, 41
US$ per night on single rooms and US$ 46 per night on double rooms (a
bed for a couple, I already checked by phone). Likely most people know
HI Hostels, but I think it's worth suggesting. The first time I went
abroad I didn't know about them.
Buenos Aires also has other hostels
<http://hostelsuites.com/hostels.php?idioma=2>. Maybe locals could
inform which are the closest to the Wikimania event place. For me
Hostel Florida seems OK.