The train tickets at Stansted being so expensive is probably down to the rail industry. A 1-week return between London and Stansted Mountfitchet (a few mins from the airport but on the main line to Cambridge and not a separate spur) costs £22.60
A 1-week return from Mountfitchet to the airport costs £4.90
A 1-week return from London to the airport costs £31.50
The reason for the price disparity is that the train company does not sell the cheaper offpeak tickets to/from the airport but sells offpeak tickets from London to Mountfitchet
I suspect that tickets to the airport are treated as a cash cow to effectively subsidise train services in rural parts of East Anglia. There are no votes to be had from foreigners, but residents of East Anglia can vote in the UK. Furthermore the only people travelling every day by train to Stansted airport can partake in the scheme for reduced fares anyway, leaving nobody to protest