Quick answer: Penang runs about EUR 45 per day mid-range - noticeably less if you eat local and walk. Cheapest window: December-March - the driest months; September-October brings the heaviest rain.
What things actually cost
char kway teow
1.80 EUR
kopi (local coffee)
0.50 EUR
assam laksa
1.20 EUR
Grab across George Town
1.50 EUR
Penang Hill funicular return
6 EUR
Blue Mansion guided tour
5 EUR
Eat well for little
Char kway teow at Siam Road Charcoal Char Koay Teow - worth the queue, about 1.80 EUR
Assam laksa at Air Itam market - sour fish noodle soup, Penang's signature, about 1.20 EUR
Nasi kandar at Line Clear - rice flooded with mixed curries, about 2.50 EUR
Cendol at Penang Road Famous Teochew Chendul - shaved ice with palm sugar, about 0.80 EUR
Hokkien mee (prawn noodle soup) at a kopitiam - about 1.50 EUR
Money mistakes to avoid
Assuming hawker legends are always open - many stalls close one or two weekdays and sell out by early afternoon; check days before you queue
Basing yourself in Batu Ferringhi for a food trip - you will spend an hour a day commuting to George Town's stalls
Renting a car for the island - parking is painful and Grab rides across George Town cost 1-2 EUR
Walking the heritage core at midday - do murals and mansions before 11am, then museums or hill in the heat
Transport without the tourist tax
From Penang International Airport a Grab to George Town costs about 4-5 EUR and takes 30 minutes. In town, Grab rides run 1-2 EUR and the free CAT shuttle bus loops the heritage core.