Sort posts by custom fields value using dropdown menu

Instead of query_posts try using WP_Query class. http://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters

This is example from codex:

$args = array(
'post_type'  => 'my_custom_post_type',
'meta_key'   => 'age',
'orderby'    => 'meta_value_num',
'order'      => 'ASC',
'meta_query' => array(
    array(
        'key'     => 'age',
        'value'   => array( 3, 4 ),
        'compare' => 'IN',
    ),
),
);
$query = new WP_Query( $args );