If I were seeking that information I would first contact three or so of the local log home builders to ask them what new-home prices are running for log homes.
Second, I would also check the two MLS's involved (Black Hills MLS (Rapid City area) and Northern Hills MLs). Since the MLS systems do not have a consistently utilized specific category to search on in order to isolate on "log" homes in a search, that would have to be done more manually. It happens that I have some special methods to search the MLS database for log home sales the past year, beyond the capability of the routine MLS system, so contact me if you'd like help with that.
Third, if you would like to know all the log homes offered for sale by owners in the past year, let me know. I could help you with that.
Hope this helps.
Prudential Real Estate