Norwegian Railways (NSB) are usually very expensive. But they offer a cheap ticket called ‘Minipris’ for 199 NOK / 299 NOK for any oneway journey inside Norway (also for changing connections, e.g. from Bergen via Oslo to Bodø) and to and from Göteborg. The Minipris tickets are limited to a number of seats in each train, and this number seems to be not very high, especially not for the 199 NOK tickets. So book early. You can easy book online on NSB.
To buy the tickets online they ask you to enter the last eight digits of your phone number. Actually it's not important if this is your real phone number, it must be just a 8 digit number you surely don't forget, because you need this number to collect the ticket from a ticket machine. And that's the problem if you travel from Göteborg to Norway: You can buy a Minipris ticket for that route, but you cannot collect the ticket outside Norway! So, if you havn't been to Norway before to collect your ticket you cannot do this route. You would have to buy a normal SJ ticket to Norway in Sweden.
