There are external power supplies available that connect directly to a disk. I would be more skeptical regarding I/O-performance. As far as I can see, there are only two SATA-ports available.
For a cheap x86 NAS, you might want to look out for the Asrock Q1900DC which was presented at CeBit this year and is supposed to come out soon.
It has 4x2ghz Intel Celeron J1900, 4 SATA2 and 2 SATA3 ports and 19V DC input, so no more need for a bulky ATX power supply (though for a NAS you need to find a way to power those hard disks).
Judging from the prices that similarly specced boards are fetching (between 50-70€), this one should be below 100€ as well.
It has 4x2ghz Intel Celeron J1900, 4 SATA2 and 2 SATA3 ports and 19V DC input, so no more need for a bulky ATX power supply (though for a NAS you need to find a way to power those hard disks).
Judging from the prices that similarly specced boards are fetching (between 50-70€), this one should be below 100€ as well.
Here is a slide from the Asrock CeBit presentation with the specs: http://4.bp.blogspot.com/-1wOiaMWiiXw/Ux7HGzZAl6I/AAAAAAAAHQ...