HZMB bridge to Zhuhai and then taxi to Zhuhai or Zhuhai North train station is likely to be quicker than going in to HK..
Hong Kong and Guangzhou are only 119 Km apart. That said Baiyun airport is a fair way North of Central Guangzhou. And it is a lot of stops on the subway from Guangzhou South station - takes about an hour (58 KM!) but is only about 10 Rmb
Guangzhou North is less than 20 KM to Baiyun. Taxi will be well under 100 Rmb. But the area is less Western friendly than the other Guangzhou stations.