I don't seem to have the links (I'll keep looking) but check the prices for the bergen-oslo train leg to see if there is any sale pricing available. It might be significantly cheaper to buy that ticket separate from the NIAN package. I did bergen-bergen with NIAN, then had a separate train ticket to Oslo, IIRC I bought a 1st class ticket for less than the standard regular class thanks to a sale.
If you haven't already, seach this forum for NIAN and you'll find a lot of information/questions from others