Quick answer: Seville runs about EUR 105 per day mid-range - noticeably less if you eat local and walk. Cheapest window: March-May and October-November - orange blossoms or golden light, avoid July-August when 40C days are normal.
What things actually cost
cafe con leche
1.60 EUR
single tapa
3 EUR
cana (small beer)
1.80 EUR
Real Alcazar entry
15 EUR
Cathedral and Giralda entry
13 EUR
city bus single ticket
1.40 EUR
Eat well for little
Espinacas con garbanzos (spinach and chickpeas) - 3-4 EUR a tapa, a Moorish-rooted Seville staple
Salmorejo (thick chilled tomato cream) - about 3.50 EUR a tapa, topped with egg and jamon
Jamon iberico tapa - around 5 EUR at a classic bar like Las Teresas in Santa Cruz
Solomillo al whisky (pork loin in whisky-garlic sauce) - 4-5 EUR a tapa
Vino de naranja (local orange wine) - about 2.50 EUR a glass, try it at a bodega in Triana
Money mistakes to avoid
Visiting in July or August - the heat regularly passes 40C and afternoons shut down, spring and autumn are a different city
Not booking Real Alcazar tickets online in advance - same-day entry often sells out completely
Booking a big dinner-and-show flamenco package - small tablaos and penas in Triana are cheaper and far more intense
Driving into the old town - lanes are barely car-width and access is restricted, park outside or skip the car
Transport without the tourist tax
The EA airport bus reaches the center in about 35 minutes for 4 EUR; a taxi runs a fixed fare of roughly 25 EUR. The old town is best on foot - it is compact and largely pedestrian - with single bus or tram tickets at 1.40 EUR when needed.