Quick answer: Hong Kong runs about EUR 110 per day mid-range - noticeably less if you eat local and walk. Cheapest window: October-December - dry, mild and the clearest skies of the year; March-April is a humid second choice.
What things actually cost
MTR ride in the centre
1 EUR
Star Ferry crossing
0.50 EUR
wonton noodle bowl
6 EUR
milk tea at a cha chaan teng
2 EUR
Peak Tram return with Sky Terrace
16 EUR
pint of beer in a bar
8 EUR
Eat well for little
Dim sum at Tim Ho Wan (Sham Shui Po) - the famous baked char siu buns, a full meal for 8-12 EUR
Wonton noodles at Mak's Noodle or Tsim Chai Kee - small but perfect bowls, about 6 EUR
Roast goose at Kam's Roast Goose (Wan Chai) - a plate with rice around 15 EUR
Milk tea and a pineapple bun at any cha chaan teng - the local diner experience, about 4 EUR total
Egg waffles from a street stall in Mong Kok - about 2.50 EUR
Money mistakes to avoid
Skipping an Octopus card - it works on every train, tram, bus and ferry and most shops; buy one on arrival
Going up Victoria Peak at midday - haze and queues; go the last hour before sunset instead
Staying only on Hong Kong Island - Kowloon has the views of the island skyline, plus cheaper food and hotels
Assuming cards work everywhere - small noodle shops and market stalls are often cash or Octopus only
Transport without the tourist tax
The Airport Express reaches Central in 24 min for about 13 EUR; bus A21 to Kowloon costs about 4 EUR. In town, the MTR plus the historic ding-ding trams cover everything, with most rides 0.50-1.50 EUR on an Octopus card.