My Assistant: Jobs with unbilled WIP - how to get the amount of unbilled WIP to display?

I use the automated notice via My Assistant called "Jobs with unbilled WIP older than 90 days" (we may have customized the length of time).  I think it would be really helpful if the report could actually display the amount that is unbilled.  (If for example, we haven't billed $10 in photocopy costs, no big deal, but if we have thousands sitting there unbilled, I want to know about it!)  Is there a way to add this information?  I tried searching to add the field but it doesn't seem to display the BL unbilled file in the choices and I'm not savvy enough to write a formula.  Please help thanks!