Sleeping : 10 - 30€ per night
Eating : 10€ for a decent lunch or diner
Drink : 3€ a pint of beer.
Avoid finding accommodation near the city center (Barrì Gotic) where prices soar. It's better to move a bit further away, like near the station "El Clot-Arago" where you will find cheap hotels. The district is well served and especially very quiet ! (Ex: Catalonia Sagrada hotel).
Otherwise, you can choose youth hostels or renting an apartment as there are a lot of options on B&B websites.
For Eating, you will find a lot of cheap tapas bars and other typical restaurants serving regional specialities.