Well, I think you could take a D-train from Beijing to Nanjing. The journey is about 8 hours and CNY359. There are several D-trains departing from Beijing South Railway Station to Nanjing everyday. Once arrive at the airport, you could take the shuttle bus Line 2 to its terminal and transfer to metro Line 4 to the train station.

Ps: it is suggested that you book a train ticket in advance, since it will be a little difficult to get one on the spot.