These days all rail travel in the UK is very variable in price, depending on how far in advance you can book and the time of day at which you can travel - avoiding rush-hours is cheaper. Best to look at the rail websites.
The cost of a train ticket will vary depending upon how far you wish to travel, the time you are travelling, whether you booked your ticket in advance, and whether you want to travel in a first class...
The price of tickets depends on where you are going, if it is round trip, if you are sleeping in a sleeper car, and how many tickets. It can be as little as 40.00 to as much as 400.00+.