First you need to distinguish between a factory configured folder and a non-factory configured folder. The former was produced as and left the factory in a folding stock, the former did not and was assembled as a folder aftermarket. The former commands a premium...how much is up to you. I recently had similar questions as the OP and researched as many completed auctions and sales as I could find. I found prices to be the following as of early 2018 with the highest prices realized on gunbroker:
- Non-factory configured blued folders are at the bottom for $800-1,000.
- Non-factory configured stainless folders are next at $900-1,100.
- Factory configured blued folders were going for around $1,000-1,200.
- Factory configured blue folders, GB models were selling for around $1,200.
- Factory configured stainless folders were selling for $1,200-1,500.
- Factory configured stainless folders, GB models were $1,500-1,800.
The problem with attempting to ascertain prices for Mini 14 folders is that many buyers fail to distinguish between factory vs. non-factory folders and pay the former's price for an example which is the latter. You need to be careful and do your homework. If you have the serial number of one that you are considering purchasing you can call Ruger and they might be able to look it up and tell you the configuration that it left the factory. They have been tremendously helpful in running S/N's for me and saved me from a few non factory folders. Note that there may be nothing mechanically or otherwise wrong with a non-factory configured folder, it simply won't have the provenance of a factory-produced folder.
It took me a while but I finally found mine - a 1985 factory produced stainless Mini 14 folder in excellent condition with the original serial numbered box, original paperwork and 5rd mag. I also have a Ruger Letter of Authenticity for it.