I want to travel from Kristiansand to Trondheim and back in the end of August. Looking up the itinerary for a random date in August shows that this will cost me around €60 for a two way ticket.
You have found the cheapest possible train ticket for that journey. Note that in order to get that price, you need to book well in advance.
There are no weekend passes for long-distance trains (in Norwegian only).
The cheapest available InterRail pass will cost you €190 (or €129 if you're under 26).
All passenger train services in Norway, except the Airport Express train, are operated by Norwegian State Railways (NSB).
There are, to my knowledge, no other ways of purchasing a Kristiansand - Trondheim train ticket.