I didn't mean to deter you from going to Norway altogether.
If you can find a cheap flight from Bergen, instead of going to Oslo first, that will help out a great deal. For a week in Norway, I would expect to spend at least 2000 NOK on places to stay and food. If you can travel on the money left over (how much is the flight, and can you find out how much renting a car would be?), I say go for it.