http://www.serversplus.com/product.asp?s=SVHEW-633724421
Deal is on for another month
Cost £199.95, fill the form in and claim the cash back (takes up to a month)
Spec of box:
Athlon II Neo N36L 1.3GHz Dual-Core CPU, 1GB DDR3 ECC UDIMM (8gb max), Internal SATA Cold Plug SATA 250GB Hard-Drive, Embedded RAID 0/1, 1x Gigabit Ethernet, 200W PSU, 1 Year Warranty
Supports 4 disks by default, but can be modified to support up to 6 (search the forum)
Works very well with unraid (been using it for 2 months now, 24/7)
Has a built in usb port inside, which you plug your unraid stick in.