Well, I am contemplating building out a large tower with some kind of RAID, and perhaps a single SSD dedicated to MC.
I am planning on running VMware on this, and then spinning up linux instances with docker (thanks to Injate).
So I was looking at around $2,000 total. I am hoping to consolidate the other servers I have (Media, File Print, Active Directory, Mumble, Ventrilo, and the two MC servers)
The MC portion of that is probably 35% (because of the SSD)... but seriously, I LOVE Minecraft, and it wouldn't be nearly as fun without you guys playing too, so don't sweat it.