Quick answer: Dubrovnik runs about EUR 110 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September-October - warm sea, open terraces and far fewer cruise crowds than July-August.
What things actually cost
espresso at a bar
2 EUR
City Walls entry
35 EUR
local beer (0.5l) in a bar
4.50 EUR
city bus ticket
1.73 EUR
Lokrum return ferry with island entry
27 EUR
seafood main in a konoba
18-25 EUR
Eat well for little
Black risotto (crni rizot) - squid ink risotto found in most konobas, around 15-18 EUR
Ston oysters - farmed 50 km up the coast and served raw with lemon, about 3-4 EUR per oyster
Peka - meat or octopus slow-cooked under an iron bell, order 2-3 hours ahead at a konoba, about 25-30 EUR per person
Burek from a bakery - flaky cheese or meat pastry, a filling 4-5 EUR breakfast
Gelato on Stradun - decent quality despite the location, about 2.50 EUR per scoop
Money mistakes to avoid
Walking the City Walls at midday in summer - there is almost no shade, go right at opening or in the last two hours before closing
Not checking the cruise ship calendar - on days with 3+ ships the Old Town is gridlocked by 10am, plan Lokrum or kayaking for those days
Eating on Stradun itself - restaurants on the main street charge double for average food, walk two alleys uphill instead
Staying inside the walls with heavy luggage - there are hundreds of stairs and no vehicle access, pack light or stay in Ploce or Lapad
Transport without the tourist tax
The airport shuttle bus to the Old Town takes about 30 minutes and costs around 10-15 EUR, a taxi is about 35-40 EUR. The Old Town is entirely pedestrian, and local buses to Lapad or Gruz cost about 1.70-2 EUR per ride.