Supply and demand. How many people make a DAY return journey from Southampton to Ashford that takes almost 4 hours in each direction? On an average day, probably none!
I'm not really sure thats relevant - it would be if there was one train that only went from Southampton to Ashford and there was nobody on it, low prices would encourage people to use it. But thats not the case - you'll use two seperate trains which will have plenty of people using the seats making shorter journeys. The holder of a cheap Southampton to Croydon ticket for example may well use a Brighton to London service - far from suffering from a lack of demand!
In contrast, how many people travel from Birmingham to Bristol each day? Hundreds, if not thousands. It is also possible to reduce the cost of Birmingham to Bristol ticket prices by buying Advance tickets or by splitting.
--- old post above --- --- new post below ---
I guess that comes down to what you think the rail networks aim should be - provision of a public service or revenue generation for private companies who have a legal obgliation to maxmise shareholder value. I accept that currently, it is the latter and not the former and that in some areas this is what drives the pricing of various ticket flows.
On the Bristol to Birmingham example we now have the somewhat perverse situation whereby XC have been allowed to reduce the size of the trains from 7-8 coaches to 4-5, causing massive overcrowding on most, and then as a result use the concept of supply and demand to increase price. At first glance it makes the position reasonable - 'Our trains are filled to bursting point, we use price to moderate demand on these services' but then when you look at why they are so full...
I'll ignore the split ticketing argument as expecting the general public to split is unreasonable.
There is a reason why that fare is so high, why the 7DS is only £69.60, and why the CDR is only £9.90.
Whats the reason? (purely out of interest, as the 7DS seems reasonable as does the CDR).