Quick answer: Chiang Mai runs about EUR 35 per day mid-range - noticeably less if you eat local and walk. Cheapest window: November–February (cool season; avoid March–April burning season).
What things actually cost
khao soi bowl
1.80 EUR
street pad thai
1.20 EUR
songthaew ride across town
0.80 EUR
one hour Thai massage
7 EUR
latte at a specialty cafe
1.80 EUR
scooter rental per day
5.50 EUR
Eat well for little
Khao soi at Khao Soi Khun Yai - the definitive coconut curry noodle bowl, about 1.50-2 EUR
Sai ua (northern herb sausage) and crackling from Warorot Market stalls - about 2 EUR for a takeaway bag
Cowboy Hat Lady braised pork leg rice at the Chang Phueak north gate night market - about 1.50 EUR
Sunday Walking Street on Ratchadamnoen Road - graze the whole length on 5 EUR of skewers, mango sticky rice and dumplings
Specialty coffee from local mountain beans at Akha Ama - lattes around 1.80 EUR
Money mistakes to avoid
Visiting during burning season - roughly late February to mid April the air fills with crop-burning smoke; check the AQI before booking those months
Picking an elephant camp that offers riding or shows - choose an ethical sanctuary like Elephant Nature Park and book days ahead
Renting a scooter without a motorcycle license - police checkpoints fine tourists routinely and travel insurance will not pay if you crash unlicensed
Turning up at temples in shorts and tank tops - shoulders and knees covered, and carry a sarong for Doi Suthep
Transport without the tourist tax
The airport is only 15 minutes from the Old City; a Grab or metered airport taxi costs 3-4 EUR. In town, shared red songthaews cost about 0.80 EUR a hop, Grab rides run 1.50-3 EUR, and scooters rent for about 5-6 EUR a day if you are licensed.