revise-search-order-by-custom-field.php

<?php

/* [http://on.fb.me/UAe3H9] */

/*
 * Posts that have the 'mc_order_key' custom field, and that their (numeric) value is low
 * will be shown first. If you wish to reverse the order, change the 'order' parameter to
 * 'DESC'. Also make sure to change the meta key.
 */
function mc_reorder_search_results( $query ) {
	/* Let's make sure we're on the search page, and running on the main query. */
	if ( $query->is_search() && $query->is_main_query() ) {
		$query->query_vars = array_merge( $query->query_vars, array(
			'order' => 'ASC', /* Deafult is DESC, set for ASC to reverse. */
			'orderby' => 'meta_value',
			'meta_key' => 'mc_order_key' /* Change this key to match yours! */
		) );
	}
}
add_action( 'pre_get_posts', 'mc_reorder_search_results' );