Quick answer: Siem Reap runs about EUR 40 per day mid-range - noticeably less if you eat local and walk. Cheapest window: November-February - dry season with cooler mornings; March-May is brutally hot.
What things actually cost
fish amok
4 EUR
draft beer near Pub Street
0.50 EUR
tuk-tuk ride in town
1.50 EUR
Angkor 3-day pass
57 EUR
fresh fruit shake
1 EUR
tuk-tuk temple day hire
18 EUR
Eat well for little
Fish amok - coconut fish curry steamed in banana leaf, about 4 EUR at a sit-down restaurant
Beef lok lak - peppery stir-fried beef with lime dip, about 4 EUR
Num banh chok - morning rice noodles with green fish curry from a street cart, about 1.50 EUR
Khmer BBQ (phnom pleung) - grill-your-own meat platter, about 6 EUR per person
Marum - training restaurant for disadvantaged youth, creative Khmer plates around 4-5 EUR
Money mistakes to avoid
Buying a 1-day Angkor pass (about 34 EUR) when the 3-day pass (about 57 EUR) is far better value even for two visits
Not agreeing a tuk-tuk day rate upfront - a full temple day should cost about 15-20 EUR, more for Banteay Srei
Giving money or sweets to child beggars or visiting 'orphanages' - both fuel documented exploitation; support training restaurants and NGOs instead
Doing temples straight through midday - start at sunrise, break at the pool from 11am to 3pm, return for sunset
Transport without the tourist tax
Siem Reap Angkor International Airport lies about 45 minutes east; a taxi into town costs around 10 EUR. In town tuk-tuks cost 1.50-2 EUR per hop, and a tuk-tuk hired for a full temple day runs 15-20 EUR.