KFC
Only kidding :-D
I don't think there is such a thing as a nice but reasonably priced restaurant in Oslo, at least not that I have come across. If I were you I'd drop into the local bookshop and browse Lonely Planet Norway (or Scandinavia) and see what suggestions it has.
The best way to reduce the price of a meal though, is drink no alcohol with it.
Onto pubs. Lot's around the City. Sorry I can't name any as I get taken to them, as opposed to seeking them out. Expect to pay NoK50+ for 1/2L of beer in even your most average pubs. If you are visiting make sure you load up on duty free spirits, and enjoy a cocktail or two before you go out in the evening.
It is a fantastic place but crucifyingly expensive. When there I tend to economise like made for a few days, then enjoy the savings I've made on other days. Visiting a supermarket (Spar, often) and buying rolls and ham and/or cheese and knocking up your own lunch is one way to make a cheap and good lunch. (There is even a Spar just behind the main harbourfront at Aker Brygge).
I hope you have a great time!
(I highly recommend the viking museaum btw)