Shows how to set up a Participants Database field that keeps a total of several other fields.

pdb-total-field-handler.php

<?php
/*
  Plugin Name: Participants Database Total Field
  Description: Maintains a total of several fields
 */

// this updates the total when the record is updated
add_filter( 'pdb-before_submit_update', 'xnau_update_total_field' );

/**
 * updates the total field
 *
 * @param array $record the submitted record data
 * @return array the data to store
 */
function xnau_update_total_field ( $record )
{
  // set this up to list the fields to sum
  // must be numeric!
  $fields_to_total = array('numeric_1','numeric_2');
  
  // name of the total field
  // should be numeric field
  $total_field = 'numeric_sum';
  
  // calculate the total
  $total = 0;
  foreach ( $fields_to_total as $field ) {
    $total += $record[$field];
  }
  
  // place the total in the data array
  $record[$total_field] = $total;
  
  // return the record data to be saved
  return $record;
}

This code is a plugin, take a look at this article which explains how to install it:

How to Install a WordPress Plugin from a Gist