For flights in Norway you can use one of the three main airlines that operate in the country. These are SAS, Norwegian and Wideroe. Wideroe has some summer campaigns, usually in the early summer, where they offer domestic flights at very reasonable prices.
They also have a campaign going for the summer that offers you unlimited flights to all destinations in Norway. for NOK 3975. You can also buy a ticket for unlimited flights in one or two of the three zones (north, middle, south). For details please go to: http://www.wideroe.no/en/tickets/explore-norway-ticket
Flights between the big cities (Oslo, Bergen, Stavanger, Trondheim) vary a lot in price. You can get a one way flight from Bergen/Stavanger to Oslo for as little as NOK199 with SAS or Norwegian but if you are unlucky you might have to pay as much as NOK800 or more.
Plan your flights in advance and you are more likely to avoid high prices. Beware that the airline Norwegian adds an extra NOK 60 each way for one piece of check-in luggage. SAS has no extra charges and the price you see is the price you end up paying. Check the websites of SAS and Norwegian to book and find flights, destinations and campaigns.