Buy.com offers the Buffalo Technology Dualie 500GB USB 2.0 / FireWire 800 Portable Hard Drive with iPod Dock
, model no. HD-PA500IBU2D, $79.99
with free shipping
($0.16/GB). That's the lowest total price we could find by $19 and the first hard drive / iPod dock combo we've seen. It features an 8MB cache.
Of note, this hard drive does not actually sync to your iPod.