You would think that if you wanted something simple like having a subtotal line with a sum and an average that this would be easily configurable in Reporting Services. Alas… Clicking on the Subtotal button magically adds a row to the matrix component. But, you cannot tell it what function to use: it will always show the sum of the numbers above. There is also just one row that can be added.
Frustrating, if you want to achieve something simple like this:
You will need to apply a few tricks to achieve the above result.