Quick answer: San Francisco runs about EUR 230 per day mid-range - noticeably less if you eat local and walk. Cheapest window: September-October - the warmest, clearest months; summer is often foggy and cold, especially near the bridge.
What things actually cost
Muni ride
2.50 EUR
cable car single ride
8 EUR
Mission burrito
11 EUR
specialty coffee
5.50 EUR
Alcatraz ticket
42 EUR
mid-range hotel double per night
200 EUR
Eat well for little
Mission burrito at La Taqueria or El Farolito - the city's defining meal, about 11 EUR
Clam chowder in a sourdough bread bowl at Fisherman's Wharf - touristy but right once, about 12 EUR
Dim sum in Chinatown - a table full of dumplings for about 15 EUR per person
Morning bun at Tartine Bakery - worth the queue, about 5 EUR
Oysters at Hog Island in the Ferry Building - about 3.50 EUR each
Money mistakes to avoid
Not booking Alcatraz in advance - official tickets sell out 2-4 weeks ahead in high season
Packing only summer clothes - fog and wind make July feel like 14 degrees; bring layers year-round
Leaving anything visible in a parked car - window break-ins are the city's most common crime against visitors
Spending all your time at Fisherman's Wharf - it is the least representative corner of the city
Transport without the tourist tax
BART runs from SFO to downtown in about 30 minutes for roughly 10 EUR. In town, walk plus Muni buses and trains (about 2.50 EUR with a Clipper card or contactless tap) cover almost everything - the hills are the only real obstacle, which is what the 8 EUR cable car ride is for.