Sorry I wasn't clearer. I thought the taxi fee might be 140CNY for a straight Beijing Airport to Mutianyu transfer.
Really don't want to pay a guy to sit around waiting for us and I've seen the local transport options link in this thread's sticky for an idea of what this option might cost.
And the "no buggy at the Great Wall" is a given, I have been pondering where best to leave it. Leaning very much towards "at the airport" as my definitive decision.