Hey, you're correct it's pretty expensive to buy food here.

If you want to try and get a feel for some of the prices you can try your luck here with Google translate -> https://kronan.is/snjallverslun (this store is much nicer and just as cheap as the self proclaimed discount supermarket Bónus)

Expensive stuff includes but is not limited to; fresh meat and fish, most non basic fruit, fresh pastries, non factory bread, meat toppings, cheese, good yoghurt, most cereal, and more. Basically anything good and healthy. It's gotten much worse in the past two years compared to the other Nordics because the icelandic aristocracy has taken a real liking to greedflation. All the fast food and restaurants are crazy expensive. Anywhere a tourist might go is also crazy expensive.

The less expensive foods might include: basic fruit like banana and some type of apples, factory made bread, cheaper types of biscuits, cheaper types of pasta and canned goods, cheaper types of yogurt. Some dried fruit and nuts. You should be ok with making sandwiches as someone else suggested.

Good luck, try not to get stuck here for too long 😅