You need to go to someplace like eBay to find the PCIe SSD's (used). The last I heard (may be way out of date), 1TB was $1000. MCE sells them but I think they're on the expensive side relative to eBay. People have said the MCE products are also used (taken out of other Apple products) but it doesn't say that and I don't know.
http://store.mcetech.com/mm/merchant.mvc?Store_Code=mtos&Screen=CTGY&Category_Code=PCIEFLASH
Performance wise, since the Mini only has 2 PCIe lanes for the SSD (vs. 4 on the 2015 Macbook Pro), you may get 40%-50% better performance, at best, from the PCIe SSD vs. a good SATA3 SSD. And yes, the SATA3 SSD can be moved to other computers with a SATA connector or standard docking stations/enclosures. You can only move the PCIe SSD to other Apple products with the same PCIe connector or use something like the OWC Envoy Pro enclosure ($110, according to OWC, it works with the PCIe SSD's in the "Current Mac Models").
For me, from a price-performance perspective, at the current prices, putting in a PCIe SSD into the 2014 Mini isn't worth the price. It's better if you do a BTO Mini with it inside. But even then, it's $200 for a 250GB from Apple vs. $85 for an Evo 850 (and you get to re-purpose the 1TB drive). For me, I was successful in putting a SSD in my 2012 Mac but I didn't look forward to doing something similar again so I just bought an external for my 2014 Mini. Maybe once the machine is out of warranty and new models are out (so the value of the current model drops), I'll do it.