AIUI, it's not illegal and there is nothing to pay if you're under the relevant limits and it's for personal use. You can import up to 6000NOK of goods (3000NOK if you've only been out of Norway for < 24 hours) and you can bring up to 10 kg combined of meat/cheese etc from EEA countries. That's quite a lot of butter. In fact, the 2nd link below is ambiguous on the point of whether dairy products other than cheese are included in the 10 kg limit, but you'd probably want to assume so to be safe. Either way, you can import a lot of butter for personal use.

http://www.toll.no/en/international/english/shopping-abroad/...

http://www.toll.no/en/international/english/shopping-abroad/...

It's not illegal, but dairy products are generally taxed heavily to allow Norwegian products to compete on price. There may be some special permits required to import, I don't know the specifics.