Yes, whether you like it or not you must actually eat, and it must be included into your travel budget. But it doesn't have to cost you all that much, and you don't have to visit a McD's to satisfy your appetite. The tips for eating on a shoestring budget in Stockholm are as follows:
Head for the falafel
All around town you will find kebab and falafel joints. They're cheap and you get a lot of food for your money. This is a grand choice for any vegan travelers as well. Thank goodness for the middle eastern influence!
Pasta and such
Pasta and other fast food can be found at 7-Elevens all around the city. The food gets weighed so you can pay exactly as much or as little as you want.
Sushi
There are countless sushi restaurants in the city, but if the price is very low (below ) the the quality is probably lacking in some department, so in those cases, choose the vegetarian alternative. That way you're always safe.
Grocery stores
It might not be a bad idea to stop by a regular grocery store when looking for food. A good one to try out is "Hemköp" under the T-Central. It's open until 9PM and is huge. You're sure to find something to eat there.
Photo: 7-eleven
Burgers and such
Finding a burger is easy, because grills can be found literally everywhere. They're usually open 24 hours or close to it so grabbing a burger is also a great alternative. They usually cost something like and often have a whole array of other things to choose from.
Pizza
The rule with pizza is simple, if it costs more than then it's too expensive.