This feature isn't supported by FM's native page number layout object. Restarting page numbering in the middle of a report is an issue, though adding a custom leading-figure to the page number as part of a text box object on the layout is easy.
I can imagine accomplishing this with a calculated result, though thinking through the rules of how to set the value would require a clear statement of one's rules for page numbering.
It could also be done, possibly simpler, with a number field in the items table, and a looping script which set the values in the found/reporting set of records as part of the report output script.
Thought I would give the work around that I ended up using..
1st I created a higher step in the hierarchy (Order) with a field that contained the series number for each system (100,200, etc)
Then created the pageNumber field in the ITEM table (=Order::seriesNumber + GetAsNumber (Get (Page Number))
The script then creates a PDF of each SYSTEM appending each PDF to the existing PDF.
End up with a perfect PDF of the whole price book in a folder on the desktop, where it can be printed or emailed to a customer.