I've said it once and I'll say it again: The best way to dine in Spain (for your money's worth) is by having the Menu del Dia (Menu of the Day).
Through my experience in Barcelona, on our way to the Bari Gotic, we stumbled upon Rosa Negra (Black Rose). The name may scare you at first, but the food will easily change your mind.
The restaurant serves Mexican fare in what seems to be not-so-Mexican surroundings. The colors of the restaurant are vibrant, tropical colors; however, they have an interesting decor piece: a bunch of dolls side by side, hanging from the wall. The dolls were created by a woman in Mexico, hence their connection to that country.
The beef quesadilla is excellent! For only 8€, it is worth every bite.