They are rare because they can't be easily replicated. You can't just take a Mini-14 stock, cut it down and make a factory (styled) folder. There were some aftermarket folders that come close to the look and feel of the factory folder. There was even a neat underfolder version that I like - I've seen those sell for about $250. Factory folding stocks go for roughly $400, when you can find them.
Ruger never sold their factory folding stocks to the public, so they had to be married to a Mini-14. The Buttler Creek stocks work, but they are ugly (IMHO). I paid about $400 for a Sage-SCAR stock for my AC-556, and then bought the parts to make it a side-folder (I don't like the AR look for the Mini-14.) The Sage stock is far more practical and comfortable than the original folder. I have put my side-folding stock safely away, but would never part with it.