Change the sort order for an archive template by a custom field.

gistfile1.php

<?php

/**
* Change the post order for listings
*
* @author Carrie Dils
* @link http://www.billerickson.net/customize-the-wordpress-query/
* @reference http://codex.wordpress.org/Class_Reference/WP_Query
*
*/

add_action( 'pre_get_posts', 'cd_listing_sort_order' );

function cd_listing_sort_order( $query ) {
	
	if( $query->is_main_query() && !is_admin() && is_post_type_archive( 'listing' ) ) {
	
	$price = (int)'_listing_price'; //convert prince to integer
	$price = $price*1; //multiply by one

	$query->set( 'meta_key', $price );
	$query->set( 'orderby', 'meta_value_num' );
	$query->set( 'order', 'desc' ); //list high to low
	$query->set( 'posts_per_page', '6' );
	}
 
}