A single hotel room (always book ahead for weekdays) should cost around NOK800 and up (special offers are common; look for them), but you can find reasonably priced lodgings in camping huts (NOK300-600, space for entire family), mountain cabins (NOK150-300 per person), youth hostels (NOK150-250 per person), and so on. The majority of them will need you to prepare your own meals, pack your own bedsheets, and wash your clothes before departing.
Consider renting an apartment, a home, or a high-quality chalet for longer visits (one week or more). Several organizations provide reservations for homes or cabins owned by farmers or other residents. This kind of lodging is often more fascinating than a traditional hotel.