Count, sum, average, min, max in Views

Views 2

Use Views Calc module: http://drupal.org/project/views_calc

Views 3

After enable aggregation those count, min, max, sum functions will be available, see below:

Take care to avoid using the 'Distinct' option in the 'Query settings' if using aggregation, it can interfere with your intended result. Removing sort orders for SUM can also solve problems, because a sort order means the results you want to sum are no longer identical.

Another way is that follow http://drupal.org/node/1370254 (the template file e.g. views-view-table.tpl.php), calculate whatever you want, then customize append the results to the html table.