The Norwegian summer is short and hectic, and people tend to congregate at the prime watering holes whenever the sun pokes through. The locals come armed with their credit cards, prepared to pay what it costs. For foreign visitors, though, be prepared to pay 10 Euro for a 0.5 liter glass on the waterfront. If you seek out a more quiet place, the prices are more humane. There is an online list of places to avoid here.
People tend to blame the taxes, but the profit margins are high. The alcohol duty on a 80 kroner beer is about 8,50, and the brewery takes 17,50. You should be able to save up a bit for a rainy day on the difference.