I believe the rare boxed action figures keep their value better unsigned because of the difficulites of authenticating the autograph should you wish to sell it later.
On the upside ... JRD used to have a lovely bold sig (don't know that he
still does though), so at least it ought to be fairly easy for it to be recognised as genuine at some future date.
I suppose the thing you have to ask yourself is ... 'will I be selling this?' ... and then do an eBay search for similar cast-signed items to get some idea on how much they've sold for recently.
Roz raised a very good point too ... the cost to have JDR sign it in the first instance ... twenty quid is twenty quid and if him autographing the figure detracts from it's value then I certainly wouldn't do it.
Does it have to be this particular figure he signs or have you another less-rare one which you could have signed instead?
I don't know how many people from the UK send stuff to the AFA to be authenticated Rabb ... I for one would be absolutely loathe to send
anything out of the UK for this to be done ... assuming I were lucky enough to own something of value which needed this service to start with!

I think the general feeling so far is have him sign it if you don't plan on selling it, but if you do want to sell it then leave it untouched.

Do the searches and see what's sold ...
