At a restaurant:
The price of a bottle of house wine (0.75l) => 20 euro
The price of a sirloin/steak meal => 11 euro
The price of a pizza => 5-6 euro
Typical food => kleftico 10.75 euro / stifado - 8.5 euro / moussaka - 8.75 euro

At a bar
The price of a bottle of beer (0.33l) => 3.75 euro
The price of a cocktail => 4.5 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) => 1 euro

Rent a small car
Rent of car for 1 day => 33 euro/day
Rent of car for 3 days => 90 euro
Rent of car for 7 days => 188 euro

Sources:
http://www.travel-rhodes.com/
http://www.rhodesguide.com/
http://www.galaxy-rentacar.com/general.php
http://www.romeo.gr/greekm.htm
http://www.ippotikon.com/