I got one of these for $529 from the B&H sale about a month ago. A week later, they dropped the price to $499 and they happily refunded me $30 for the asking.
I use mine for an iTunes and file server, it runs 24/7 and replaces a base 1.4ghz/4gb 2014 Mini. I split the 128gb SSD from the fusion drive, since it's a server and only has about 30gb on the system drive (all my files and media are on a 4tb USB3 external disk). Works very well and was a great deal for 500 bucks IMO.
Only thing is, I was running El Capitan on the old Mini and decided to upgrade the new Mini to Mojave. It seems to run fine, but filesharing appears to be much slower under Mojave. I haven't taken the time to really investigate this, but a quick test only showed about 80MB/sec writing to the 128gb SSD over the network, and I'm pretty sure I saw speeds closer to 100MB/sec with the old Mini on El Capitan. The internal SSD clocks about 700MB/sec, so no problems there.
Regardless, for 500 bucks these 2014 mini's are great, as long as you understand its limitations and don't expect something it can't deliver. Apple was selling the same model for $1000 less than a year ago, and they are still selling refurbs for something crazy like $750 IIRC. It should be more than adequate for a server IMO.