I figured it out.  If you export a report where the columns are collapsed, the exported columns with the totals in them are calculated from columns that are not exported, so the formula can't export.  So in these cases, excel will export the number instead of the formula (it really doesn't need to do that for the $ over budget and % over budget columns, since they are calculated based on the totals columns, but for whatever reason it exports the number there too). 

SO TO SOLVE IT you have to export the report with the columns expanded and then hide the columns you don't want in your exported report.  Kind of a pain in the butt, unfortunately.