Quick answer: Queenstown runs about EUR 150 per day mid-range - noticeably less if you eat local and walk. Cheapest window: December-February for lake days and hiking, June-August for skiing - March-May brings golden larches and fewer crowds.
What things actually cost
flat white at a cafe
3 EUR
Fergburger with cheese
9 EUR
pint of local craft beer
7 EUR
Skyline Gondola return ticket
35 EUR
Orbus bus ride with Bee Card
1.10 EUR
Milford Sound coach and cruise day tour
130 EUR
Eat well for little
Fergburger - the famous burger institution, order the Ferg Deluxe and expect a queue, about 9 EUR
Ferg Baker next door - venison or steak pies for a cheap lunch, about 5 EUR
Rata - Josh Emett's native-bush-themed fine dining in a laneway, mains around 28 EUR
Green-lipped mussels at a lakefront bistro on Steamer Wharf - a New Zealand must-try, about 13 EUR
Central Otago pinot noir by the glass at a Gibbston cellar door or town wine bar, about 8 EUR
Money mistakes to avoid
Booking Milford Sound for your last day - weather cancellations are common in Fiordland, schedule it early in the trip so you can rebook
Renting a car just to stay in town - parking is scarce and expensive and the center is walkable, rent only for day-trip days
Underestimating the sun - New Zealand UV is extreme even on cool days, wear sunscreen on every hike and lake cruise
Visiting only in late December-January without booking months ahead - accommodation prices roughly double in peak summer
Transport without the tourist tax
Queenstown Airport in Frankton is 8 km from the center - the Orbus route 1 runs every 15 minutes for about 2-5 EUR while a taxi costs 20-30 EUR. The town itself is fully walkable and Orbus buses with a Bee Card reach Frankton, Sunshine Bay and Arrowtown for about 1.10 EUR a ride.