Do not use a calculated table field. Its unnecessary and wastes space.
It can also fail at times without you being aware of that.
See
http://allenbrowne.com/casu-14.html for more details
Use an aggregate (totals) query to calculate averages. It will automatically allow for (exclude) null values.
To test that, create a totals query using the totals button on the ribbon and include a number field including some bills.
In the totals row, change it to average and run. Note the result.
Go back to query design view and in the filter criteria enter 'is not null'.
Run again. You'll get the same result