Quick answer: Athens runs about EUR 110 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April-June and September-October - warm but not brutal, July-August regularly tops 35C.
What things actually cost
freddo espresso
3 EUR
gyros pita
4 EUR
90-minute metro ticket
1.20 EUR
Acropolis entry (summer rate)
20 EUR
taverna dinner with wine
18 EUR
0.5l beer at a bar
4.50 EUR
Eat well for little
Gyros or souvlaki pita - 3.50-4.50 EUR, the benchmark cheap meal, try Kostas near Syntagma
Greek salad with barrel feta - 7-9 EUR at any taverna
Loukoumades (honey doughnuts) - 5-6 EUR a portion, Krinos is the classic stop
Freddo espresso - about 3 EUR, the default summer coffee
Full taverna dinner in Psyrri with house wine - 15-20 EUR per person
Money mistakes to avoid
Climbing the Acropolis at midday in summer - go at 8am opening or the last two hours, and book timed tickets online
Eating at places with photo menus and touts on Adrianou Street - walk two blocks into Psyrri or Koukaki instead
Treating Athens as a half-day layover to the islands - the museums and food deserve two full days minimum
Accepting taxi price 'quotes' - insist on the meter or use the FreeNow app
Transport without the tourist tax
Metro line 3 runs from the airport to Syntagma in about 40 minutes for 9 EUR, or the X95 express bus costs about 5.50 EUR. The center is walkable and the metro covers the rest - a 90-minute ticket is 1.20 EUR.