Quick answer: Barcelona runs about EUR 95 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May–June or September (warm, beach + fewer crowds).
What things actually cost
espresso at a bar
1.40 EUR
menu del dia lunch
13 EUR
cana (small beer)
2.80 EUR
metro ride with T-casual
1.20 EUR
Sagrada Familia entry
26 EUR
glass of vermouth
3.50 EUR
Eat well for little
Pan con tomate with jamon - the default start to any meal, around 3-5 EUR at most tapas bars
Bombas at La Cova Fumada in Barceloneta - the potato-meatball bomb was invented here, about 2.50 EUR each
Pintxos on Carrer Blai - skewers at 1.50-2 EUR apiece, grab a plate and pay by the toothpick
Menu del dia - three-course weekday lunch with wine at neighborhood restaurants for 12-15 EUR
Vermut hour - a glass of house vermouth with olives and chips for about 3-4 EUR, best before Sunday lunch
Money mistakes to avoid
Eating on La Rambla - overpriced frozen paella; walk five minutes into El Born or Poble-sec instead
Not booking Sagrada Familia and Park Guell online days ahead - same-day tickets regularly sell out
Showing up for dinner at 19:00 - kitchens are dead until about 21:00, do a vermut or pintxos first
Keeping your phone in a back pocket on the metro and La Rambla - pickpocketing is the city's main crime, use a zipped front pocket
Transport without the tourist tax
From El Prat airport take the Aerobus (about 7.25 EUR, 35 minutes) or metro L9 (about 5.50 EUR) to the center. In town walk plus metro with a T-casual 10-ride card (about 12 EUR, roughly 1.20 EUR per ride).