If you like wine, please aware a bottle of whit wine or red wine, 75ml, is from 2000-kr up (13 euro up). Get some stocks from duty free would be a option. In Iceland the real liquor shop is called vinbuð. The "beer" in supermarket is only 2.5% alcohol.
Dinning out, of course, a bit expensive. Lunch in gas station is from 3-4 euro for a soft drink with a hot dog. Dinner is 15 to 20 euro up in general restaurant.
If you like to the coffee shop, a cup of coffee is about 2.5 euro.
One of our residency artist, Jeanne Morrison did a research on the price in supermarket. Please visit her blog here.