Quick answer: Lima runs about EUR 55 per day mid-range - noticeably less if you eat local and walk. Cheapest window: December–April (summer, less grey).
What things actually cost
menu del dia lunch
3.50 EUR
ceviche in Miraflores
10 EUR
pisco sour
5 EUR
Metropolitano bus ride
0.80 EUR
Uber Miraflores to Barranco
2.50 EUR
espresso
2 EUR
Eat well for little
Ceviche at a neighborhood cevicheria - classic leche de tigre and fresh corvina for 8-12 EUR at lunch
Lomo saltado - soy-seared beef stir-fry with fries and rice, about 6-8 EUR in Miraflores
Anticuchos in Barranco - grilled beef-heart skewers from evening street grills, about 3-4 EUR
Menu del dia at a local lunch spot - soup, main and drink for 3-4 EUR away from tourist streets
Pisco sour at a Barranco bar - the national cocktail, about 5 EUR
Money mistakes to avoid
Ordering ceviche at dinner - it is a lunch dish; fish arrives in the morning and the best cevicherias close by late afternoon
Hailing taxis on the street - unregulated and prices are invented; use Uber or Cabify where rides cost 2-5 EUR
Underestimating traffic - a 10 km hop can take an hour at rush hour, plan sights by district and allow buffer time
Expecting sunshine from May to October - the garua fog keeps the coast grey; it is not rain, but pack a light jacket
Transport without the tourist tax
From Jorge Chavez Airport take an official airport taxi or app car to Miraflores (10-15 EUR, 30-60 minutes depending on traffic). In town use Uber or Cabify for 2-5 EUR per hop plus the Metropolitano bus (about 0.80 EUR) along the main north-south corridor.