Quick answer: Dubai runs about EUR 140 per day mid-range - noticeably less if you eat local and walk. Cheapest window: November-March - warm and sunny without the extreme heat; June-August regularly exceeds 40 C and life moves indoors.
What things actually cost
metro ride with Nol card
1.50 EUR
shawarma
3 EUR
karak chai
0.50 EUR
Burj Khalifa standard ticket
45 EUR
abra creek crossing
0.25 EUR
desert safari tour
60 EUR
Eat well for little
Shawarma from a Deira or Satwa counter - about 3 EUR
Pakistani grill at Ravi Restaurant in Satwa, a local institution - full meal for about 8 EUR
Karak chai from a roadside cafeteria - about 0.50 EUR
Emirati breakfast (balaleet, chebab) at Arabian Tea House in Al Fahidi - about 12 EUR
Grilled kebabs at Al Ustad Special Kabab near Al Fahidi metro - about 9 EUR
Money mistakes to avoid
Trying to walk between sights that look close on the map - blocks are huge and sidewalks vanish; use the metro or taxis
Not pre-booking Burj Khalifa tickets - walk-up sunset slots sell out and cost far more
Ignoring local norms - public drunkenness and aggressive PDA can cause real trouble; drink only in licensed venues
Visiting in July or August expecting beach weather - 45 C afternoons make outdoor plans miserable
Transport without the tourist tax
The metro red line runs from the airport to Downtown and the Marina for about 1.50-2 EUR with a Nol card. The metro plus occasional metered taxis (start about 3 EUR) covers almost everything; driving is easy but parking downtown is not.