It is quite expensive to eat in most of the restaurants of the city. Average meal may cost from 15-30 USD, there are some tips you may use to eat on budget in Stockholm:
Avoid Expensive Restaurants: 1st of all avoid eating on most restaurant on Gamla Stan and Drottinggatan, main shopping street of the city.
Prefer to Buy Grocery: Prefer self cooking, grocery for a week may cost you around 400 SEK (60$); one time average meal would cost over 15$.
Budget Restaurants: Try only small stalls on the sides of street, these restaurants are pretty economical ranging from 3-10$ from a hotdogs to a burger.
Water Refill Bottle: May keep a refill water bottle, tap water in Stockholm is safe to drink so save upto 3$ for a small drinking water bottle.