Quick answer: Krakow runs about EUR 75 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September - mild weather, long evenings and fewer crowds than midsummer.
What things actually cost
espresso at a cafe
2.50 EUR
0.5l local beer
3 EUR
plate of pierogi
7 EUR
20-minute tram ticket
1 EUR
zapiekanka at Plac Nowy
3.50 EUR
Wawel Castle state rooms entry
8 EUR
Eat well for little
Pierogi (mixed plate) - 6-8 EUR at a pierogarnia like Przystanek Pierogarnia
Zapiekanka at Plac Nowy - 3-4 EUR, the classic half-baguette late-night snack
Obwarzanek (street bagel) - about 0.80 EUR from the blue carts, breakfast on the go
Zurek (sour rye soup) - 4-5 EUR, often served in a bread bowl
Milk bar (bar mleczny) full lunch - 5-7 EUR for a soup and main, a Polish institution
Money mistakes to avoid
Using 'zero commission' exchange kantors near the Main Square - the spread is the commission, compare rates or use an ATM in PLN
Not booking Auschwitz-Birkenau weeks ahead - entry slots sell out, especially guided tours in English
Paying in euros when offered - always pay in zloty, card terminals' currency conversion rates are poor
Staying only on the Main Square - rooms cost more and Kazimierz is better for evenings anyway
Transport without the tourist tax
The train from Krakow Airport to the main station takes about 20 minutes and costs around 4 EUR. The center is walkable; trams cover everything else with 20-minute tickets at about 1 EUR, and Bolt/Uber are cheap for late nights.