I did this trip just last month. We took the train from OSL to BGO, and then flew back on Norwegian Air Shuttle (for our dates, it was much cheaper than SAS). The train ride was great (didn't do NIAN, but had enough great views). We were 2 people travelling and got the MiniPris tickets in advance for about $85 for both of us, which was far less than the normal price for one person. You can specify to pick up your tix. onboard the train, meaning that the conductor hands them to you, so it's really easy. I think the flight back to OSL was about $65/each, and we timed it with our connection back to the USA so that we didn't have to take the expensive airport train back to OSL, so it worked out pretty well.

BGO is nice...you can see most of it in half-a-day. Take the furnicular up the mountain for some good views. Expensive country overall, though!