Modifying meta box fields in a child theme

modify-metabox-fields.php

<?php
/**
 * Registering meta boxes
 *
 * Based on these metaboxes and a global metabox variable, but can be modified to suit others:
 * @link http://www.deluxeblogtips.com/meta-box/docs/define-meta-boxes
 */
// Add new metaboxes - hook into after the metaboxes have been defined, but before they have been registered
add_action( 'admin_init', 'jdn_modify_metabox_fields', 1 );

/**
 * Modify exist metaboxes to include a new text field
 * @author Joshua David Nelson, josh@joshuadnelson.com
 **/
function jrn_modify_metabox_fields() {	

	$prefix = 'prefix_';
	global $meta_boxes;// or whatever variable/class, etcetera is housing your metaboxes
 
	if( !is_null( jdn_find_in_array( $meta_boxes, 'field_id', 'field_term' ) ) ) {
		$key = jdn_find_in_array( $meta_boxes, 'field_id', 'field_term' );
		$fields = $meta_boxes[ $key ][ 'fields' ];
		$new_field = array(
			array(
				'name'	=> 'Custom Price',
				'desc'	=> '',
				'id'	=> $prefix . 'custom_price',
				'type'	=> 'text',
				'clone'	=> false,
				'std'	=> '',
			)
		);
		$meta_boxes[ $key ][ 'fields' ] = array_merge( $fields, $new_field );
	}
}

/**
 * Function to search for a metabox field and return the key
 * @author Joshua David Nelson, josh@joshuadnelson.com
 * @link http://stackoverflow.com/questions/8102221/php-multidimensional-array-searching-find-key-by-specific-value
 **/
function jdn_find_in_array( $array, $needle, $term ) {
	foreach( $array as $key => $value ) {
		if ( $value[ $term ] === $needle )
			return $key;
	}
	return null;
}