If you mean the amount that is calculated.

View the Config of the profile that you wish to change the amount in.
Then in the left hand menu, navigate to -> Log Data -> Log Filters.
one of the pre-canned filters is labeled
"Compute bandwidth cost"

Edit this filter and when the "Edit Filter" window appears you need the 2nd tab along. "Filter".

If you need to change the currency symbol then you have to manually edit the profile .cfg file (c:\Program Files\Sawmill for IronPort\LogAnalysisInfo\profiles\*.cfg)