Basses are worth what people will pay for them, which reflects multiple factors, just like any consumer good. Jimmy Coppolo has a great reputation as a luthier (even though there has been the odd QC issue, as Paulie noted - in fact I owned one of the basses he mentioned and should note it was a prototype). His basses have a certain cache and strong advocates, as do Ken Smith's, Foderas, Alembics and do on. I mean, is a used Wal really worth £5k? Different strokes for different folks. All of which is stating the obvious!
As Jack says, maybe try playing one? They are very nice instruments indeed. I don't own one any more but I would also (like Jack) love to have another play on the antigua KBP5 we each owned at one point (I sold it to pay for a double bass)