At a restaurant:
The price of a bottle of house wine (0.75l) => 20 euro
The price of a sirloin/steak meal => 10-11 euro
The price of a pizza => 6 euro
Typical food =>  moussaka - 9 euro / kleftico 11 euro

At a bar
The price of a bottle of beer (0.33l) => 3.5 euro
The price of a cocktail => 4.5 euro

At the local supermarket
The price of a bottle of water (1l) => 1.1 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 => 35 euro/day
Rent of car for 3 days => 95 euro
Rent of car for 7 days
=> 195 euro

Sources:
http://www.kefalonia.net.gr/en/default.asp
http://www.oskars.gr/
http://www.kefalonia.net.gr/en/showcat.asp?CatID=26&ScatID=4
http://www.kefalonia.net.gr/en/showcat.asp?CatID=6&ScatID=33
http://www.kefalonia.net.gr/en/showcat.asp?CatID=6&ScatID=34