If you're going to have wine in a Norwegian restaurant, I wouldn't recommend the cheapest option. The duty adds about £30 to each bottle, so you may as well go for something decent at £70-80 rather than spend £50 on something basic. They generally don't import rot-gut since it's all going to be so expensive anyway!
That having been said, I actually picked up a 'bargain' bottle of Dom Perignon in (the old) Oslo duty-free - £50 when the best price I could get it for in the UK was over £80.