Wow that seems high. In any case, typically CGC Signature Series will hold the most value because they are witnessed and sealed. It's not an opinion that the signature is authentic.
That said, most signatures are NOT witnessed. In that case, third party authentication is the next best thing.
Thanks! This book at 9.2 goes for around 300-400 slabbed and unsigned, so it's a matter of whether the verified signature to me is worth an extra 500 :(