Hike on the west coast

Here is the hostel in Stavanger: http://www.hihostels.no/no/Vandrerhjem/Fjord_Norge/Stavanger_Mosvangen/ Price NOK 225-395 + bedlinen, breakfast etc. - multiply with 14-21 and you have an estimate -add some hotels for the places without a hostel, a hostel only open in summer or where it is fully booked by a group.
I calculate (at least) some NOK 6000-8000 for 3 weeks accommodation
Stick to NOK - Norway does not use some version of "$".

Food - calculate twice the amount you would have used at home, if home i France as your profile says.