hey! I am going to be island hopping from the 7th-20th of sept, trying to figure out the same thing...i was told to bring prolly around $75-$80 a day...are you gonna be hopping then?
I went on the GIH Tour in June. I spent more cash than I thought I would on drinks going out every night...so have the cash on you. I remember a lot of the drinks were between 8-10 euro each. If you like beer there is a good cheap local beer called Mythos that everyone drank on my tour and the Greek gyros are so yummy and cheap only like 3 euro too! Also if you really like jewelry shopping Santorini is great!