It looks like you've made up your mind. When we did a similar trip in August, we flew from Tromso to Trondheim, mainly because we didn't want to burn up two days on transportation. We later took the Hurtigruten from Trondheim to Alesund and from Alesund to Bergen. If you want to save money, you can look into booking as a deck passenger. You don't get a cabin. There is a luggage room to store your gear, and some ships have lockers for your valuables. We did that on our two voyages, but each was only about 12 hours.
--Dav
