Hi Sara,
You will save about $57 per adult by booking this yourself and more for Seniors (67+ 50% discount) children (6-17, 50% disc.) and students (25% discount with student ID).
The only disadvantage of booking yourself is that you need to book tickets in several places and know the schedule of the various transit legs to make all the connections smooth. The trains can be booked 3 months ahead through the Norwegian Railways - www.vy.no/en (Oslo-Myrdal-Flåm & Voss-Bergen). The boat ride through the Nærøyfjord between Flåm and Gudvangen can be booked through www.visitflam.com. The Gudvangen-Voss bus can't be booked ahead. You can buy this from the bus driver upon arrival (I've never heard of anyone not getting on). You need to pay for the bus ticket in cash, 115 NOK.
Booking the trip through Fjord Tours is the easiest option and it includes all the tickets (including the bus) but there are no Senior or Youth discounts. If you decide to book the Nutshell yourself, I recommend using Fjord Tours website first to get the schedule, so you know what time to book each leg. The Norway-in-a-Nutshell chapter in Rick's Scandinavia and Norway guidebooks also list some sample schedules.
