Such are never "backup" since the hard disks are prone to the same failures as our internal drives. A fine second copy but never trusted. Proof is on these forums.
As to price that will vary by location and what's on sale. I saw the usual 250GB drive and case for sale at 99USD but unless you have an Apple, why firewire?
Bob

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic