I am sure you will find lots of restaurants where the main course is below 290 NOK. Even in Oslo, at Aker Brygge you can find restaurants that would cost you below 300 NOK for a main course. (This is for pasta, Burger etc). Steak and such would probably cost a bit more.
Many restaurants don't have their own website, and I am 100% sure that you will find something good and affordable if you just take a walk in Trondheim