At a restaurant:
The price of a bottle of house wine (0.75l) => 30$ [20.4 euro]
The price of a sirloin/steak meal => 11.95 euro
The price of a pizza => 6 euro
Typical food => Kleftico 10.50 euro / moussaka - 8.5 euro / stifado - 8.5 euro

At a bar
The price of a bottle of beer (0.33l) => 5$ [3.4 euro]
The price of a cocktail => 7$ [4.75 euro]

At the local supermarket
The price of a bottle of water (1l) => 1.2 euro
The price of a can of Coca Cola (0.33l) => 0.6 euro
The price of a can of beer (0.33l) => 0.9 euro

Rent a small car
Rent of car for 1 day => 30 euro/day
Rent of car for 3 days => 82 euro
Rent of car for 7 days => 180 euro

Sources:
http://www.santorinigreekrestaurant.co.uk/menu.htm
http://www.santorini.co.nz/menu.htm
http://www.sphinx.gr/categories-menu-en.php
http://www.santorini.com/
http://www.santorini.net/home.html
http://www.santorini.com/rentals/coolcars/