I always hate it when they tell you to make an offer, because you don't know what they're thinking, and you don't want to insult them. I'd much rather have them set their own price.
Having said that, I'm basically a cheapskate, and would probably start low--maybe $500 or $600 or so, and see what happens. You can always go up.
As has been said here before--it's worth whatever price two people can agree on.
