A 250GTO went in Japan a few years ago for £10M, I think that is the highest publicly known about, but the word is that one went for even more in a private sale.
Prices have dropped now, but it would be interesting to see what Nicks would go for with its 250 GTO plate, but good on him, he is never likely to sell it to some one that would probably buy it as an investement and never use it.
tut