A run doesn’t have a set value. Instead, its value changes from one match to the next and arguably even fluctuates over the course of a single Test. This is why it’s important not to pay too much attention to averages.
In Tests, the value of a run is defined by how many of them can be scored for the loss of 20 wickets. If a team can score 800 for the loss of 20 wickets, runs are plentiful and less valuable. If a team can only score 300 with their 20 wickets then a single run contributes a great deal more towards a team’s cause.
Today, England lost six for 50-odd and then India lost seven for 117. That’s all that needs to be said about Alastair Cook’s 122 and Kevin Pietersen’s 186.