Quick answer: Prague runs about EUR 70 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April–May or September–October.
What things actually cost
half litre of Pilsner in a local pub
2 EUR
goulash with dumplings
8 EUR
90-minute public transport ticket
1.60 EUR
flat white in a specialty cafe
3 EUR
Prague Castle main circuit ticket
10 EUR
tram ride with 30-minute ticket
1.20 EUR
Eat well for little
Svíčková - beef sirloin in creamy root-vegetable sauce with bread dumplings, about 8-10 EUR at a proper hospoda
Goulash with dumplings at Lokál - Pilsner Urquell tank beer and classic Czech cooking, mains around 8 EUR
Chlebíčky (open-faced sandwiches) at Sisters near Dlouhá street - the Czech deli tradition, about 2 EUR each
Vepřo knedlo zelo - roast pork with dumplings and sauerkraut, the Sunday classic, 8-9 EUR
Tank Pilsner in a local pub - a half litre costs about 2 EUR in Žižkov or Vinohrady, double that on Old Town Square
Money mistakes to avoid
Using street currency-exchange kiosks - several are notorious for terrible rates and fake 'commission-free' signs; withdraw from bank ATMs (decline the conversion) or pay by card
Hailing taxis on the street near tourist sights - overcharging is common; use Bolt or Uber instead
Never leaving the Royal Route - the Old Town Square to Charles Bridge corridor is a fraction of the city; cross to Vinohrady, Letná or Vyšehrad and prices halve while crowds vanish
Buying trdelník as 'traditional Czech' - it is a tourist invention; fine as a snack but do not queue 20 minutes or pay over 3 EUR for it
Transport without the tourist tax
From Václav Havel Airport take bus 59 to the metro and ride into the center on one 90-minute ticket for about 1.60 EUR, or the Airport Express to the main station for about 4 EUR. The center is very walkable and the tram network fills every gap - a 24-hour pass costs about 5 EUR.