From Zhangjiang BBS:

黑车的报价如下：
早晨3元，有雪，暴雨，车少，抓车5元。特殊时期，10元
晚上2元，暴雨，车少，抓车3元。有雪，特殊时期5元
愿坐就坐，不坐就算。

Illegal taxi-van fares are as follows:
Morning is RMB 3; snowy, rainy days with few vans are RMB 5; special occasions RMB 10.
Evening is RMB 2; snowy, rainy days with few vans are RMB 3; special occasions RMB 5.
If you willing then take them, if not then just forget about it.

I've found that late at night when the buses stop running, the morning rates kick in again.