If you make it your self it will cost you 2.7~3.2K
what you need:
> A laptop 2.5" HDD
you can also get a 3.5" one if you need more space and size dosnt matter
Companies: samsung, seagate
Cost: 2300 [80gb 3.5"] to 23K [720GB perpendicular recording Behemoth

]
> One external casing
Cost: 500[indian made VIP ones] to 1200.
>a +head screwdriver, your hands, and your head
Cost: 20bucks for the screw driver, your head, priceless.