The distance between Barking and Southend, and the distance between High Wycombe and Marylebone is roughly 50 km for both journeys but the rail providers charge different prices for off peak trips.
A C2C train from Barking station to Southend central is £8.50, that is about 15p per kilometre. A trip from High Wycombe to Marylebone is more expensive.
A journey of a similar distance, 50km is more expensive on the Chilterns railway, it costs about 26p per kilometre. It is so uncompetitive that it is cheaper to take an Uber if you have 4 passengers.
My question is why? Why is it cheaper to travel 50 km in the C2C train network then the children railways? Is it because the Chiltern railways still use diesel trains? Is it because there are two train lines that go to Southend so they face more competition? I don't know the answer, I only know that every rail network has different prices for equivalent distances.