You can find factory folding stocks for $350-$400; that's just what they sell for. Yes, there was an aftermarket stock made that look very similar to the factory Ruger folder. You would have to look really close to tell the differences. These clone stocks are even harder to find and they still sell for about $250 if you were to find one. They are pretty good quality, though.
There was also an underfolder made for the Mini-14. They look like the factory folder, except the stock folds under like an AK-47 rather than to the side. These are pretty good quality and would give you a similar "look" to the factory folder. I've seen the underfolders sell for $150-$200, and it's been a number of years since I've seen one for sale.
One other alternative is a SCAR (Sage) stock. It's made of milled aluminum and you can add a folding stock to it (comes with AR type stock)