Quick answer: Cusco runs about EUR 50 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-September - dry season with clear skies and cold nights; June has the Inti Raymi festival but also peak crowds.
What things actually cost
menu del dia lunch
3 EUR
coca tea
1 EUR
taxi across town
1.50 EUR
Machu Picchu entry
38 EUR
Boleto Turistico (16-site pass)
32 EUR
market fruit juice
1.50 EUR
Eat well for little
Menu del dia (soup, main, drink) at a local lunch spot - about 3 EUR
Lomo saltado (beef stir-fry with fries and rice) - about 5-7 EUR in a mid-range restaurant
Fresh fruit juice at San Pedro Market - about 1.50 EUR for a huge glass
Alpaca steak in a San Blas restaurant - about 10 EUR
Cuy (roast guinea pig, the festive classic) - about 15 EUR if you are curious
Money mistakes to avoid
Doing a big trek or Rainbow Mountain on day one - give yourself 24-48 hours to acclimatize first
Leaving Machu Picchu tickets for last minute - entry slots and train seats sell out weeks ahead in dry season
Drinking alcohol the first night at altitude - it hits much harder and worsens soroche (altitude sickness)
Buying 'baby alpaca' scarves from street stalls - most are synthetic; buy from reputable shops and check the feel
Transport without the tourist tax
The airport is inside the city - an official taxi to the center costs about 4-5 EUR. The historic center is best on foot, taxis across town cost 1.50-2 EUR, and shared colectivos to the Sacred Valley run 2-4 EUR.