I think it might help if you accumulate the data in your spreadsheet source first.
For example to use the SUMIF function to sum up all the Invoice$ for each Salesperson.
But there might be a better solution within mashzone..
If you want to do all steps into mashzone I suggest the following:
1 - Define your excel source.
2 - "Copy" it
3 - To be done twice : delete invoice column > Most recent then oldest date with salesperson dimension
4 - combine the two "copies" and you should have one line per salesperson and two date columns "most recent" and "oldest"
5 - here you calculate gap between the two dates to find the number of days - see http://www.ariscommunity.com/users/andretogo/2011-07-31-calculation-time-between-two-dates-different-lines
6 - From your copy operator (see "2"), do the following:
6.1 - delete date column
6.2 - aggregation operator with salesperson as dimension and invoice as "sum"
6.3 - result - total invoice per salesperson
7 - combine the result of 5 and of 6.3
You have the following columns: salesperson / total invoice / number of days (maybe more...) and then you can use the arithmetic operator to calculate the average invoice per day.
Hope this will do. this is surely not the only option.