is there any bus which travels from Urumqi to Nanning & what about ticket price? days?

As I know, there is no direct bus, train or air service between two places.
You are suggested to take a direct train from Urumqi to Zhengzhou (33 hours around), where you can transfer another high speed train to Nanning (10 hours around). With the soft sleeper, it may cost you about CNY1500 around.

Compared taking a flight, it isn’t cheaper. If taking a flight, you need to transit in Xian or Chongqing.