Quick answer: Hanoi runs about EUR 40 per day mid-range - noticeably less if you eat local and walk. Cheapest window: October–November or March–April (mild, dry).
What things actually cost
bowl of pho
1.80 EUR
egg coffee
1.20 EUR
glass of bia hoi
0.40 EUR
banh mi
1.20 EUR
Grab ride across town
2 EUR
Temple of Literature entry
1.20 EUR
Eat well for little
Pho bo at Pho Gia Truyen, 49 Bat Dan - the benchmark beef pho, about 1.80 EUR, morning queues move fast
Bun cha at Bun Cha Huong Lien - grilled pork with noodles at the spot Obama ate, about 2 EUR
Banh mi at Banh Mi 25 - crusty pate-filled baguette for about 1.20 EUR
Egg coffee at Cafe Giang - whipped yolk over dark coffee, invented here, about 1.20 EUR
Bia hoi at the Ta Hien corner - fresh draft beer on plastic stools for about 0.40 EUR a glass
Money mistakes to avoid
Freezing mid-street while crossing - walk slowly and steadily and the scooters flow around you; sudden stops cause accidents
Hailing random street taxis - meters get rigged; use Grab where a cross-town ride costs 1-3 EUR
Only eating in tourist restaurants - the best food is at single-dish street stalls; busy stalls with locals are the safe ones
Changing money at the airport - rates are poor; use gold shops on Ha Trung street or ATMs in town instead
Transport without the tourist tax
From Noi Bai Airport take Bus 86 to the Old Quarter (about 1.50 EUR, 45 minutes) or a Grab car for 10-12 EUR. In town walk the Old Quarter and use Grab bikes or cars, with most rides costing 1-3 EUR.