Quick answer: Sydney runs about EUR 130 per day mid-range - noticeably less if you eat local and walk. Cheapest window: October-November and March-April - warm, less humid and outside the summer school-holiday crowds.
What things actually cost
flat white
3 EUR
meat pie
4 EUR
ferry to Manly
5 EUR
schooner of beer at a pub
5.50 EUR
Opal daily transport cap
11 EUR
Opera House guided tour
26 EUR
Eat well for little
Flat white at a Surry Hills cafe - about 3 EUR
Meat pie from Harry's Cafe de Wheels in Woolloomooloo - about 4-5 EUR
Fish and chips on the beachfront at Manly - about 12 EUR
Dumplings in Chinatown around Dixon Street - about 8 EUR a plate
Barramundi and a schooner at a pub bistro - about 20 EUR
Money mistakes to avoid
Underestimating the sun - UV here burns in 15 minutes even on cloudy days; sunscreen is non-negotiable
Swimming outside the red-and-yellow flags - rips are real; patrolled zones exist for a reason
Buying tourist transport passes - just tap a contactless card on the Opal network, fares are capped daily
Staying only around Circular Quay - the character is in Newtown, Surry Hills and the beach suburbs
Transport without the tourist tax
The airport train reaches the center in 15 minutes for about 13 EUR (the station access fee is most of it). Around town, tap a contactless bank card on trains, buses and ferries - the Opal daily cap is about 11 EUR, and the Manly ferry doubles as a harbour cruise.