Generally Copenhagen is quite expensive, but there are some good cheap
Peder Oxe (http://www.pederoxe.dk)
35DKK on burgers every Monday
Café Stella (www.cafestella.dk)
10% discount on all food when you show student card.
Riz raz (www.rizraz.dk)
Cheap good buffets –Vegan friendly.
Dalle Valle (www.cafedallevalle.dk)
Close to Nørreport station with ½ price on all food every Saturday, Sunday, Monday and Tuesday.
Café Zusammen (www.cafezusammen.dk)
Close to Forum station with ecological food and cocktails. Cheap brunch buffet every Sunday.
Amager Sushi (www.amager-sushi.dk)
Sushi take away on Amager. Big pieces and reasonable prices.
Restaurant Vita (www.restaurantvita.dk)
Restaurant Vita is one of the oldest in Copenhagen and is situated in an old pharmacy from the 17th century. At Vita's you can taste original Danish food such as old-fashioned roast duck served with prune, apple, homemade cucumber salad, red cabbage and brown and white potatoes.
Thomas Sandwich (http://thomassandwiches.dk/)
Several years in a row Thomas Sandwich has been chosen the best one of its kind in Copenhagen. At Thomas' you can get eight different kinds of sandwiches for 45 DKK.
At Café Katz you can get good food and drinks for reasonable prices. In the summertime there are tables outside the Café just next to the canal and the Castle of Christiansborg which gives a "hyggelig" atmosphere. A good advice is to reserve a table if you plan to eat there.
Café Alma (www.cafealma.dk)
Café Alma is to find in a basement on Islands Brygge. It has a great atmosphere and a lot of young people go there to eat or drink a good cup of latte.