Quick answer: Buenos Aires runs about EUR 55 per day mid-range - noticeably less if you eat local and walk. Cheapest window: March–May or September–November (spring/autumn).
What things actually cost
empanada
1 EUR
coffee with two medialunas
3 EUR
subte ride with SUBE card
0.40 EUR
steak dinner with wine at a parrilla
18 EUR
glass of Malbec at a bar
3 EUR
10-minute taxi ride
4 EUR
Eat well for little
Bife de chorizo (sirloin) at a neighborhood parrilla - a serious steak with fries runs 10-15 EUR; book weeks ahead for Don Julio
Empanadas - baked beef or ham-and-cheese pockets, about 1 EUR each; order a mixed half dozen
Choripán - grilled chorizo sandwich with chimichurri from a parrilla stand, 2-3 EUR
Helado at Rapa Nui or Cadore - Argentine ice cream rivals Italy's; a cone with dulce de leche costs about 3 EUR
Café con leche with medialunas - the standard breakfast in a historic cafe, about 3 EUR
Money mistakes to avoid
Changing money without checking rates first - pay by foreign card (which gets the favorable MEP rate) or compare the blue rate before exchanging cash; the official bank rate can cost you significantly
Showing up for dinner at 7 pm - restaurants are empty or closed; locals eat from 9:30 pm and peak around 10:30 pm
Keeping your phone out on the street or at sidewalk tables - opportunistic snatching is the most common crime; stay casual but discreet
Treating La Boca like a normal neighborhood - stick to the Caminito blocks and take a taxi there and back rather than walking from San Telmo
Transport without the tourist tax
From Ezeiza Airport take an official remis or app taxi to the center for 20-25 EUR (about 45 minutes) - avoid unofficial drivers in the arrivals hall. In town get a SUBE card: the subte and buses cost roughly 0.30-0.50 EUR per ride, and taxis are plentiful and cheap by European standards.