from Beijing to ShenZhen by train, how much does it cost and how long will it take?

There are three trains between the two cities.
One high speed train, G71 at around 08:00 and it costs about 936.5RMB for second class seat and 1479.5RMB for the first class seat.
There are two common train, K105 and T107. The fare is around 434.5 to 464.5RMB based on different bunks.

Oh, forget to tell you that the G-train takes about 10 hours and 16 minutes.
The T107 takes about 24 hours and the K105 takes about 29 hours.