I visited those two cities without the Nutshell tour last year. Flew into Tromso (from Oslo) and paid for two northern lights chase tours (under $200 each tour per person). Later took the Bergen Railway round trip from Oslo (train only), but there was no fjord boat trip, train tickets cost ~$80 (bought about 10 weeks in advance, overnight train back to save on a hotel). What also deterred me from the Nutshell tour was having to carry all your luggage with you as you move from train to ferry to bus, etc.