Hi c_madera!
I would strongly suggest reservations if you'd like to see what you're getting before getting there (via a website, that is) and to be sure such places have a room available before arriving. There's nothing worse than lugging your luggage door-to-door to find a place to sleep. You can pay anywhere from, say, 20 Euros to 60 Euros depending on if it's a single or double and the hostel itself.
I can personally recommend the following and have stayed at them all. You can find feedback about all of them by using the SEARCH link above/right:
Hostal López -
Hostal Aresol -
Hostal Cervelo -
Hostal Oporto -
Hostal Chelo -
Hostal Dulcinea Many MANY MANY people have stayed at and reviewed many of the hostels in Madrid (and Spain's larger cities) which have websites. You can find most of those reviews by using the SEARCH link above/right which I mentioned before.
Have a GREAT time in Spain! Saludos, MadrdiMan
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