Iy you're buying it as an investment because it's signed by Cliff, bear in mind what will happen to its value IF Cliff is prosecuted for being naughty. If you want it because you're a Cliff fan you could probably get it cheaper without autograph.
I must say that I am puzzled by people who will pay extra for an autograph; if you didn't hold it out to him personally for him to sign it seems valueless to me.