The total is easy and is just done with the Analysis menu item - Totals -> Show Column Grand Totals
The average is trickier. What is the average of? is it the avg by each category or the avg of each page or ?? Designing the average value should be done on another page and then added to a dashboard with the crosstab view. That's the fastest and easiest way to maintain.
There may be cases when it is possible
to have a single view cross-tab layout
with two distinct Custom (Grand) Totals
using a (self)UNIONed datasource table,
a Custom Total aggregate calculation
and a trick with hiding some value(s)
of a [Table Name] blue pill on Rows.
Please find the attached.
Hope it could be of some help.