Worse comes to worst, you could buy all 3 cables and return the ones that don't work. Even if you don't return them, you're still out $150 vs. $800..
Are you sure it is only the cable? Appears there were video problems also with some logic boards, but they should be covered under an Apple repair program. You will still need the s/n, though. See http://www.apple.com/support/ibook/faq/
I would give them a call directly and see what they say about costs.
If you have anything from the previous owner, like the sales receipt or the original packaging, these may have the s/n so you don't have to open the machine.