Food can be sorted in different categories:

Groceries: basics to cook yourself or to warm up. All also offer some catering. Prices are equivalent to Norway, except fresh items and dairies that are very expensive.
Cafés propose sandwiches, daily dish and soup for less than 100NOK.
Hamburgers, pizzas, daily dishes in restaurants and pubs range between 120-250NOK.
Taverns, regular restaurants provide regular dishes from 200 to 500NOK.
Gastronomic restaurants offer several courses meals, from 850NOK and more with additional wine pairing.

Drinks in Svalbard is much cheaper than in Norway, but still expensive: count around 70NOK for a beer.

More and more offer vegetarian dishes, sometimes vegan (but it is far from being the main fashion), as well as lactose-free and gluten-free. Think about mentioning your dietary restriction when you book your tours and order your food.