In reply to gritstone ryan:
you might want to check march isn't to late in the season?!
We recently flew with Norwgian airlines, this was the cheapest at the time and was about £100 each then we pre booked a hire car and drove oslo to rujkan. You will have to drive or get a bus. We then stayed at
http://www.rjukanhytte.com
Flying with Norwegian air we had 20kg baggage and 10kg hold allowance but for an extra few quid I would consider paying for extra baggage to take food from home and split the cost between you, included teabags and a flask of tea etc. Food is mega expensive even in the supermarket. I took a massive bag of porridge for 4 of us for breakfast as it was the best idea I think I had! We also ate together rather than individually which kept the cost down.
The accom was pretty good but showers and water where available from a seperate building. Showers required tokens and for how much it cost us to 10nok per person to shower each day.
hope some of this helps