Barcelona is moderately expensive by Spanish standards. Budget travelers spending €60-80/day can cover a hostel dorm (€18-28), a menu del dia lunch (€11-14 including starter, main, and drink), tapas-hopping for dinner, and Metro travel. Sagrada Familia (€26-36 with towers) is the big ticket expense — worth every cent.
Mid-range travelers (€120-170/day) can stay in good hotels, eat at proper restaurants, visit multiple Gaudí sites, and enjoy beach clubs. The Hola BCN travel card (2-5 days, unlimited metro and bus) is good value for sightseers. Many museums offer free or reduced entry on the first Sunday of the month.