A single hotel room always book ahead for weekdays should cost you from around NOK800 and up special offers are common and cheaper, but you can find reasonable cheap lodgings in camping huts NOK300-600, space for entire family, mountain cabins NOK150-300 per person, youth hostels NOK150-250 per person, etc. Most of these will require you to make your own food, bring your own bedsheets, and wash before leaving.
For longer stays one week or more consider renting an apartment, a house or a high quality cabin. Several agencies offer reservations on houses or cabins owned by farmers or other locals. This type of accommodation is frequently more interesting than a standard hotel.