As ever, that $850 is a lot more than a decent 2 bay with well priced BYODrives. Like 200-300% more.
But I have some Apple stock so please throw money at them.
QNAP TS-269L without drives is $485, by the time you add a couple of 2TB enterprise level drives, it's actually more than the MacMini is... QNAPWorks quotes $1430 with WD drives...
http://www.qnapworks.com/TS-269L.asp
And - well it is Intel based, but it's an Atom compared to an IvyBridge Core i7 quad for the MiniServer I linked earlier.
I'll admit up front though - QNAPWorks is a VAR, so you get a fair amount of markup, but you also get a tested/verified good solution - if you buy the box itself, and provide a couple of WD Red 2TB drives, it's more reasonable - around 180 for the drives, which takes us to $665
Alt approach - base Mini which is a Core i5 dual core @ 2.5Ghz...
A base MacMini is $599 USD, and while it only has a single 500GB drive, and then add a LaCie 4TB USB3.0 drive for 200 more... OSX Server app is 20 bucks in the AppStore... Thunderbolt GiGe NIC 30 bucks...
And it does have more functionality than most NAS's with a full blown ServerOS... the one thing missing is MySQL, which is a free download, and a DCHP daemon, which is also free via MacPorts or Fink.
So...
QNAP with Drives - $1430 from QNAPWorks - ready to run
QNAP - BYOD - add two 2TB Reds - $665
MacMini Base with Drive, OSX Server, extra NIC - $849
Not the 200-300 percent difference you speak of...