external harddrives these days aren't very expensive…. i got a 200GB maxtor external HDD for $99 at fry's. the only problem is that it runs on USB 2.0, has no firewire, and there is no existing software/driver that lets it work on a mac.
so i guess that's no help... but try going to fry's or microcenter. they usually have externals for $1-$1.5 per GB. one time i even saw a 250GB one for $150. i'm not sure about the brand, though.
fry's have an online store called outpost.com. here are the search result for USB/firewire-ready extHDD.
the one that i bought looks similar to this one: http://shop2.outpost.com/product/4770479
only it's 200GB and only costs $99. but beware. these types are not OSX compatible. you might wanna try the seagate ones like http://shop2.outpost.com/product/4596327 (free shipping! wheeee)
if the website doesn't tell you if it's OSX compatible or not, i'm sure you can google up some reviews on the harddrive by people who have bought it.