Hi,
sorry for late response. There is no such built-in option, but it's pretty simple to solve this.
You can define display callback for this field, then in this method, sum values into some static variable (static class variables are hold per request). So only displayed rows will be summed.
Then after displaying your RB table (probably in body method of your moduile) you have to display some text with summarized value.
public static function net_value_display($r, $nolink, $desc) {
$value = $r[$desc['id']]; // your field value
MyModuleCommon::$sum += $value;
return $value;
}
print "Sum of net value is: " . MyModuleCommon::$sum;
Regards,
Adam