I visited the website for Rafter J. The chances of those cabins being availible are somewhere between dream on and not in your lifetime.
Their least expensive cabin is a mere $58/nightly. And the campground claims they don't raise the rates during the Rally. For the record, most campgrounds charge this much at Rally time for a couple pitching their tent on a 15' x 15' piece of ground in a hay field.
I'll tell you this. If one of the cabins is availible, grab it. Grab it within the hour. These are cabins in a full time campground with a swimming pool/whirlpool and full amenities.
The typical Sturgis area campground has ZERO amenities. Port a potties/outhouses

are provided as bathrooms, and showers with varying degrees of warm water are usually availible. Thats it.
Cabins anywhere within 75 miles of Sturgis generally go for $200 - $300/night and up.
Supply & demand. The American way.