Any place in Barrio Húmedo (wet 'hood!...) you know, it's full of tiny bars where you can have a corto* starting at 0,80€ (i think the most expensive is about 1,20€). With each drink (wine, beer, grape-juice, coke, etc) you get a tapa (actually pretty big) for free. You can have dinner going to 5 places (what should be something like 5 euros all dinner).
*corto = 1/2 caña = 1/4 pint... what means... corto = 125 ml of beer aprox.