I wish to travel on the bullet train from Shanghai to Changsha - what is an indication of probable cost please?

There are only two bullet trains between the two cities, D105 and G576/G577.
The second class seat for D105 is 257.5RMB for the second class seat and 323.5RMB for first class seat.
The second class seat for G576/G577 is 468.5RMB for the second class seat and 694RMB for the first class seat.

Thank you Gray for your reply - I am slightly puzzled by the vastly differing prices for the 2 trains - if they are both bullet trains, why is one practically twice as expensive as the other? Is it a question of time taken for each of the trains perhaps? Much obliged for your info
Brian