Quick answer: Berlin runs about EUR 120 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September - warm, long days and fewer crowds than mid-summer.
What things actually cost
espresso at a cafe
2.50 EUR
doner kebab
6 EUR
0.5L beer in a bar
4.50 EUR
single AB transit ticket
3.80 EUR
museum entry (Neues Museum)
14 EUR
mid-range dinner main
16-20 EUR
Eat well for little
Currywurst at Curry 61 near Hackescher Markt - the classic quick bite, about 4 EUR
Doner kebab at Mustafa's Gemuese Kebap - expect a queue, about 6 EUR
Schnitzel at a traditional Kneipe like Max und Moritz - about 16 EUR
Turkish breakfast in Kreuzberg or Neukoelln - big shared spread, about 12 EUR per person
Craft beer at a Neukoelln taproom - about 5 EUR for 0.4L
Money mistakes to avoid
Not validating your BVG ticket in the small stamping machine before boarding - inspectors fine 60 EUR regardless of excuses
Assuming cards work everywhere - many bars, bakeries and doner shops are still cash only, carry 20-30 EUR
Planning museums on a Monday - several major ones are closed, check hours before you go
Trying to see the whole city from one base - Berlin is huge, group sights by district to avoid living on the U-Bahn
Transport without the tourist tax
From BER airport take the FEX or S-Bahn to the center in 30-40 min for about 4.40 EUR (ABC ticket). In town the U-Bahn, S-Bahn and trams cover everything - a single AB ride is about 3.80 EUR and a day ticket about 10.60 EUR.