Quick answer: Bali runs about EUR 50 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April–June or September–October (dry, less humid).
What things actually cost
warung meal (nasi campur)
2.50 EUR
scooter rental per day
6 EUR
flat white in Canggu
2.50 EUR
Bintang beer
2 EUR
one-hour Balinese massage
8 EUR
Gojek short ride
1.50 EUR
Eat well for little
Nasi campur at a local warung - rice with a spread of dishes for 2-3 EUR, the everyday Balinese meal
Babi guling (suckling pig) - about 3-4 EUR a plate in Ubud, Ibu Oka is famous but any busy warung is great
Nasi goreng or mie goreng - 2 EUR at warungs, triple that at beach clubs for the same dish
Smoothie bowls in Canggu - 4-5 EUR, the cafe-brunch scene is genuinely strong
Sate lilit - minced seafood satay on lemongrass sticks, around 2.50 EUR a portion
Money mistakes to avoid
Underestimating travel times - 30 km can take 2 hours in traffic, do not plan Uluwatu and Ubud in one day
Renting a scooter without an international driving permit or helmet - police fines and voided travel insurance are common
Never leaving the Canggu-Seminyak strip - the island's best scenery is in Sidemen, Munduk and the east
Drinking tap water or cheap arak cocktails - stick to bottled water and branded spirits, methanol poisoning is a real risk
Transport without the tourist tax
From the airport agree a fixed taxi price or use Grab/Gojek - Canggu costs about 15-20 EUR and can take over an hour in traffic. On the island rent a scooter (5-7 EUR per day) if you are experienced, otherwise hire a private driver for about 45 EUR per day.