I have an AFL Melbourne premiership commemorative card that is numbered 405/400, however I have had little to no interest in it since I picked it up, which was in 2009 I think.
Personally I think error cards when they are higher end ones, would be worth either less, I mean think of the arthur beetson legend signature, without his sig, I just dont see the value in it. However low end inserts or even base cards are potentially worth much more(as a percentage).
For sig cards, to have them unsigned, I sort of think they lose the appeal, if the signature was of a diff player, that would be an interesting situation. Imagine Shane Webke signed the Arthur Beetson card instead of his own card (they were both redemption sigs that year)... Although interesting, I think it would only really increase in value if the player signing is of higher interest than the original player, i.e. Arthur Beetson signed Webke's card
Of course, thats just my 2c