Born in Barcelona, and I think you could be interested:
-Always look for bars that offer 'menú'. That means a starter, a main course and dessert/coffe (pretty likely both) for around 8-9 ?. That includes one drink, or if u ask for 'vino con gaseosa' (wine and soda), they usually leave the bottle.
Chinese restaurants are even cheaper (none of my friends from abroad ever belive it), but for the 'menú' you get three courses -starter, a plate of rice or noodles and a main plus coffe or dessert for....the cheapest I know is 4, 50 (four, yeah!), tought usually is around 6 ?.
Beware that sometimes chinese can have a week day menu and a weekend menu (more expensive -around 9 ?...avoid tourist traps and try to go to working areas (anywhere up the Ramblas past Pza. Catalunya, left of right would do nicely.
Bon apettite!

