how much tip should I give?

Bill, you may give 10 to 70 RMB per day as suggested.

I live in China for 10 years. He's probably not expecting a tip at all (tipping is still uncommon in china) but if you feel like tipping him, around 10 RMB is a good number.
Bear in mind that the rough salary for a driver is only around 2500- 3500 RMB per month!

I know its prob too late now but $950 is an astronomical price to pay for a lift from the airport. I don't know which airport it is or where your hotel is. But a taxi from the south airport, to the very north of BJ would be at most 200RMB ($40). From the normal BJ airport to somewhere in the middle of the city should cost around 100RMB ($20)