I want to know best cheapest way to travel from Nanjing to Xiangyang? Or by train?

There are two methods for you.
There is only one train between the two cities and it is K282/K283 and it takes about 15 hour and 30 minutes. The fare is around 272.5RMB per person for hard sleeper.
Or you can go to the local Zhongyangmen Bus Station to take the buses at 11:40 and 16:30 to Xiangfan (Xiangyang and Xiangfan are the same place). The bus fare is around 278RMB per person. The bus takes about 11 hours and 30 minutes.
Hope these information can help you.