Charging rent for a shop on home property

[Kim Van Natter]

I'm looking into buying a fixer upper house which has a large shop on it- 36' x 48' x 16' (could store a boat or other vehicles, could be a workshop space, etc.). Also slightly rural, 30 minutes from Coeur d'Alene, one of the biggest towns in north Idaho, however the housing demand is booming all around CDA and this area is quickly becoming a bedroom community. -Should be a reasonable demand for a shop rental.

I'm a total noob, so I've been learning how to calculate rent costs by just looking through rental listings. How would I go about finding a reasonable rent cost for a shop when I can't find any other listings in our area to cross compare?