Been a while since I last used a private car with driver service (days before the 2008 Olympics) RT Mutianyu & Summer Palace (from 7am to 6:30pm). The car was not a bimmer, only a recent model full-sized Hyundai (smoke free and air). Comfortable nonetheless.
I negotiated with the driver (who parks outside the hotel everyday) after knowing what it might cost if booked through hotel. Final deal was 500 RMB, payable at the end of trip.
The entire experience was good. Driver spoke a few words of English. He understood spoken English more than he can speak. Polite, punctual and courteous. He mentioned jade museum once. After I told him I was not interested in shopping, he never brought up any shopping idea again. Invited him to join us for lunch, but he gratefully declined as he already had it during our hours on the Wall.
A day or two before you need the car, try talk to some drivers outside the Pen and get some sense of what it costs. The hotel is ripping you off big time, based on what they quoted you.