Hey Steve.

You would not be able to use the sum field since it is an expression and not
a field.

Since your quantity and price vary per line you can not sum each and multiply.

The only way I coudl see it working is if you only had a set number of rows
each time and you could manually calculate it but this is probably not
realistic.

I would add a total field to your repeating table.
You can have an expression that references a field so just because it is an
expression box it does not mean there is not a field.  Otherwise design the
MS template and see how they do it.

Another possible option would be to have a running total field.  So as each
line was totaled it would add the new amount to the running total.  That
could work if they would not be deleting rows.