In reply to Glen Staniforth: A few tips:
1. Look for flights with Norwegian.com (I fly from Edinburgh but they fly from other UK airports) to Oslo Gardermoen, great baggage allowance (20Kg with an additional 20Kg for a very small extra charge)and a lot nicer than Ryanair who fly to Oslo Sandefjord (which is like flying to London Oxford) and the airport is tiny. Overall Norwegian work out a lot cheaper than Ryanair - this February I paid £110 return compared to a total of £170 that Ryanair would charge to fly from Prestwick and the latter flight allowed less baggage. Ryan air also fly to Oslo Rygga but that is quite a way to the East of Oslo thus making a longer drive to Rjukan. Allow 3.5 hours for the dive to rjukan, it isn't that far (150Km) but norwegian roads are not always good and you will have (hopefully) snow and ice on the road.
2. I stay at Rjukan Gjestegard Hostel in the centre of town, about £30 per night including an as much as you can eat breakfast (and they provide poly bags so you can make up your lunch from the breakfast table (typical scandinavian breakfast - meat, bread, cheese, fish, eggs salad, cereal, jam etc etc)
3. Food prices in the local supermarket are OK
4. Beer and wine is extortionately expensive and most has to be bought from the state controlled off licence.
5. Take your duty free booze allowance with you, as you are travelling outside the EU you can take a litre of spirits and it usually costs a few bob more than a 750ml bottle which is the allowance in the EU.
6. Shop around for hire car prices, they are expensive but I saved £100 for a car equivalent to that hired by some of my friends.
Hope these points help.