Quick answer: Porto runs about EUR 80 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May–June or September (warm, fewer crowds).
What things actually cost
espresso at a counter
0.80 EUR
pastel de nata
1.30 EUR
francesinha
11 EUR
glass of port
4 EUR
metro ride in the center
1.40 EUR
lunch menu do dia
9-11 EUR
Eat well for little
Francesinha - Porto's meat-stuffed sandwich drowned in beer sauce and cheese, 10-12 EUR at Cafe Santiago or Lado B
Bifana at Conga on Rua do Bonjardim - spicy marinated pork sandwich for about 3 EUR
Pastel de nata warm from Manteigaria or Fabrica da Nata: about 1.30 EUR each
Mercado do Bolhao - the restored market hall, eat sardines, cheese and oysters at the counters for 5-10 EUR
Port tasting at a Gaia lodge like Taylor's or Grahams - flights from 15-20 EUR with cellar tour
Money mistakes to avoid
Not booking Livraria Lello ahead - buy the timed ticket online or you will queue an hour on the street
Driving into the center - the streets are narrow, one-way and parking is scarce; arrive by train or metro instead
Doing only the Ribeira strip - the riverside restaurants are the weakest and priciest; walk ten minutes uphill for better food at half the price
Packing only for sunshine - the Atlantic wind is real, evenings are cool even in July, bring a layer
Transport without the tourist tax
Metro line E runs from the airport to the center in about 25 minutes for around 2.50 EUR including the reusable Andante card. The center is walkable but steep; single metro or bus rides cost about 1.40 EUR and the historic tram to Foz is a scenic 3.50 EUR.