Quick answer: Budapest runs about EUR 90 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April-June and September-October - warm days, thermal baths without summer crowds.
What things actually cost
espresso at a cafe
1.50 EUR
0.5l draft beer
2.50 EUR
langos at the market
4 EUR
single metro ticket
1.10 EUR
Szechenyi Baths weekday entry
28 EUR
mid-range dinner main course
12 EUR
Eat well for little
Langos (fried dough with sour cream and cheese) - around 4-5 EUR at the Great Market Hall upper floor
Goulash soup - 6-8 EUR at a traditional etterem like Getto Gulyas
Chimney cake (kurtoskalacs) - 3-4 EUR from street stands, best fresh and cinnamon-dusted
Toltott kaposzta (stuffed cabbage) - 9-11 EUR, a heavy classic worth one dinner
Craft beer in a ruin bar - 3-4 EUR for 0.5l, cheaper than most EU capitals
Money mistakes to avoid
Exchanging money at airport or Vaci utca exchange booths - rates are poor, use ATMs and always choose to be charged in HUF, not EUR
Hailing street taxis - use the Bolt app or the official Fotaxi rank to avoid overcharging
Showing up at Szechenyi Baths at midday on a weekend - go at opening on a weekday and book online
Paying restaurant bills without checking - some tourist-area places add a service charge and hope you tip on top
Transport without the tourist tax
The 100E express bus runs from the airport to the center in about 40 minutes for roughly 5.50 EUR. In town, walk plus the metro and trams - a single ticket is about 1.10 EUR and a 24-hour travelcard about 6.50 EUR.