I can build one for under $100, just buy a cheap 2.5 inch 500GB external enclosure, and a cheap USB hub
leave one end of the external enclosure open for easy removal of the 2.5 in drive inside of the enclosure, glue the enclosure to the back of the ipod dock, then glue the hub to the bottom of the dock/ hard drive
you just saved $150 by doing it your self (most people who have a ipod generally have the dock since it comes with it so no need to worry about the dock)
while it wont look as nice, it will do the same job for less money
you can get a 2.5 in external 500GB drive for under $75
you can get a 8 port USB hub for less than $2 from ebay
you can get a 12 pack of super glue for under $1 from ebay
the ipod dock comes with your ipod
build it your self and save