Stayed here about a year ago:

http://www.portaldelsurba.com.ar/

I paid 28 pesos per night for a four-share with ensuite. Not sure about current rates. Great building, close to subways, well kept.

And if you end up in room 204, you'll have a balcony overlooking Hipolito Irigoyen. Not bad for a hostel. I believe the property also has private rooms.