Cheap hotels in Barcelona?

Hey, me and my friend are travelling to Barcelona in June for the Sonar music festival.Being young we haven't got much money to spend, so we are looking for a verrrry! cheap hotel or hostel to stay in.Either in the city centre or on the outskirts would be fine but things like bus and train stations would have to be close. Were looking to spend up to �50 a night for the room, but obviously the cheaper the better lol, thanks :)