Hi!
I strongly recommend Residencia Agora. It's the students' residence at the university of Barcelona.
It's bright, roomy, clean, quiet, with a friendly welcome, a short walk to the metro and right on the camino.
I just spent two nights there with my family. 134 euros for four persons for two nights(4 single beds), breakfast included (buffet style with an impressive array of hot and cold food for all tastes).
Average cost: 17 Euros per person per night with breakfast. Best deal in Barcelona, bar none.
I booked through Booking.com since it gives you free cancellation up to the previous day of arrival.
Hurry, they go fast.
Once you hit the road, plan to stay at the Monastery in Monserrat. Free accommodation for pilgrims.
Have a good one,
Jean-Marc