Hey there, what's the easiest and cheapest way to get from HK airport to Yangshuo.

No, there is no transport running from HK airport to YS at that time. You'd better stay ovenright in HK. You can take bus A21 to Hung Hom area and find a hotel to stay. The next morning, you can take East Rail Line from Hung Hom to Lo Wu. Then go to pass through Luohu Border, you will get into Shenzhen. Then walk to Qiaoshe Long-distance Bus Station to take bus to YS. Well, in this way, it takes a long time. In fact, the easiest way is by air from HK to Guilin, then take bus from Guilin to YS. You may meet some travel people who understand English on the journey.