Quick answer: Florence runs about EUR 135 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April-May and September-October - mild and walkable; July-August is hot, crowded and expensive.
What things actually cost
espresso at the bar
1.30 EUR
gelato small cone
3 EUR
Uffizi ticket (high season)
25 EUR
lampredotto panino
5 EUR
trattoria dinner with wine
25-30 EUR
tram or bus single
1.70 EUR
Eat well for little
Bistecca alla fiorentina at Trattoria Mario or Osteria dell'Enoteca - sold by weight, about 50 EUR per kg; one steak feeds two
Lampredotto panino from Nerbone or a street trippaio cart - about 5 EUR, the classic Florentine worker's lunch
Schiacciata sandwich at All'Antico Vinaio - about 7 EUR, expect a queue at lunch
Gelato at Gelateria dei Neri or La Carraia - about 3 EUR for a small cone
Ribollita (bread and vegetable stew) at a traditional trattoria - about 9 EUR
Money mistakes to avoid
Turning up at the Uffizi or Accademia without a timed reservation - book official tickets days or weeks ahead, especially April-October
Eating on the main squares - walk 10 minutes to the Oltrarno or Sant'Ambrogio and pay roughly half for better food
Driving into the ZTL restricted traffic zone - cameras issue automatic fines; park outside the center and walk
Cramming Pisa, Siena and Chianti into one day - pick one day trip and give Florence itself the time it deserves
Transport without the tourist tax
The T2 tram runs from Florence airport to Santa Maria Novella station in about 20 minutes for 1.70 EUR. The center is compact and mostly pedestrian - you will walk almost everywhere; buses/trams (1.70 EUR single) only matter for Piazzale Michelangelo or Fiesole.