Quick answer: Mexico City runs about EUR 70 per day mid-range - noticeably less if you eat local and walk. Cheapest window: March–May (dry, warm).
What things actually cost
taco al pastor
0.80 EUR
metro ride
0.30 EUR
specialty coffee in Roma
2.50 EUR
Uber across town
4 EUR
Anthropology Museum entry
4.50 EUR
comida corrida set lunch
5 EUR
Eat well for little
Tacos al pastor at El Vilsito or Los Cocuyos - about 0.80 EUR per taco, best after 9pm when the trompo is going
Churros at El Moro - open since 1935, churros with hot chocolate for about 4 EUR
Comida corrida at a fonda - three-course set lunch for 4-6 EUR, how the city actually eats at midday
Tostadas at Coyoacan market - ceviche and tinga tostadas for 1.50-2 EUR each
Contramar - the famous tuna tostadas and grilled fish, book ahead, about 30-40 EUR per person
Money mistakes to avoid
Ignoring the altitude - at 2,240 meters take day one slow and drink more water than usual
Hailing taxis on the street - use Uber or Didi instead, they are cheap, tracked and the standard advice
Not booking Frida Kahlo museum tickets online days ahead - they sell out and the walk-up line rarely moves
Cramming Teotihuacan and Coyoacan into one day - they are on opposite ends of the city, give each its own day
Transport without the tourist tax
From the airport take an authorized taxi or Uber to Roma or Condesa for 8-12 EUR, about 30-45 minutes. Around town Uber is cheap (3-5 EUR across town) and the metro costs a flat 0.30 EUR - avoid the metro at rush hour with luggage.