Quick answer: Naples runs about EUR 95 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April-June and September-October - warm and clear, ideal for Pompeii and ferry days; August is hot, humid and holiday-crowded.
What things actually cost
espresso at the bar
1.20 EUR
pizza margherita
5.50 EUR
sfogliatella
2.50 EUR
metro or funicular single
1.30 EUR
Pompeii entry
18 EUR
cuoppo of fried seafood
6 EUR
Eat well for little
Pizza margherita at L'Antica Pizzeria da Michele or Sorbillo - 5.50-7 EUR, expect a queue
Sfogliatella riccia warm from Pintauro or Attanasio - about 2.50 EUR
Cuoppo (paper cone of fried seafood or fritters) in the centro storico - about 6 EUR
Pasta alla genovese (slow onion and beef ragu) at a family trattoria - about 10 EUR
Espresso standing at the bar - about 1.20 EUR; it comes short, strong and pre-sugared unless you say 'amaro'
Money mistakes to avoid
Using Naples only as a Pompeii base - the centro storico and the Archaeological Museum deserve two full days on their own
Carrying your phone loose in hand on the Circumvesuviana or in dense crowds - use a front pocket or zipped bag
Renting a car for the city - traffic is intense and parking scarce; trains, funiculars and ferries cover everything
Doing Pompeii without water, a hat and a downloaded map - the site is vast and shade is scarce
Transport without the tourist tax
The Alibus shuttle links Capodichino airport with the center and the port in 20-30 minutes for about 5 EUR. The centro storico is best on foot; singles for metro, funiculars and buses cost about 1.30 EUR, and Metro Line 1's art stations are worth riding for their own sake.