Avinor do a weekly landing card for about £60 which allows you to use any Avinor airport for "no" charge for the whole week. Doesn't include Gardemoen though
Most airports are Avinor (Bergen, Stavanger, Trondheim, Kristiansund, Bodo, Hammerfest etc....).
Norway is expensive, a taxi fare from Trondheim airport to town - 30km - is about £70. But the SAS bus service is very good. I bought a round of B52's in a club in Oslo once, big mistake !