Quick answer: Rio de Janeiro runs about EUR 75 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-October - dry season, mild temperatures and lower prices; December-February is hot, crowded and expensive around New Year and Carnival.
What things actually cost
metro ride
1.20 EUR
fresh coconut on the beach
1.50 EUR
caipirinha at a kiosk
3 EUR
Sugarloaf cable car
30 EUR
churrascaria dinner
22 EUR
Uber across Zona Sul
4 EUR
Eat well for little
Feijoada (black bean and pork stew, traditionally on Saturdays) - about 10-12 EUR at a neighborhood restaurant
Picanha at a churrascaria like Braseiro da Gavea - about 20-25 EUR per person
Acai bowl at a juice bar (suco) counter - about 4 EUR
Pao de queijo and fresh juice for breakfast - about 3 EUR total
Caipirinha at a beach kiosk on Ipanema - about 3 EUR
Money mistakes to avoid
Taking your phone out on the beach or empty streets - carry a cheap decoy wallet and keep valuables at the hotel
Not booking the Christ the Redeemer train for early morning - clouds and crowds build by 11 am
Hailing street taxis at night - Uber is cheap, tracked and the standard choice for locals
Judging distances by the map - traffic is heavy, so cluster sights by neighborhood per day
Transport without the tourist tax
From GIG airport, an Uber to the Zona Sul beaches costs about 10-15 EUR; the premium BRT bus is cheaper but slower. In town, the metro (about 1.20 EUR per ride) covers Copacabana to downtown, and Ubers rarely cost more than 3-5 EUR per hop.