That is a very nice trip up the coast. Hope you are going to enjoy it. I had a trip to NorthCape planed, but then I was so stupid to fall down from a ladder and broke my left arm the day before I had to leave.
Stupid world....
But back to topic: If you are trying the Stavanger - Hirtshals ferry, then it gets cheaper the faster you order the ticket. I often take the Oslo - Frederikshavn on the way home, because it sails in the night, and then you can get a good nights sleep
cheers
p.s.
Sorry about the spelling