I agree with moondog - 500 is the price from central beijing. My parents were here last August and I asked 10+ cabs at different times of day -- None would go lower. Look for a Sonata without the annoying driver cage. If you don't speak chinese, no problem- just say what section of the wall you want to go to (Mutianyu is a relatively good mix of proximity/peace) - and write down 500 RMB and 8:00 a.m., on your hotel card so he'll know where to pick you up.
the hotel car was twice this price, and not much bigger than a Sonata.