Exclude pages by menu order

Declare the following function in your functions.php

function wpse58346_wp_list_pages( $pages, $r ) {
    foreach( $pages as $key => $page ) {
        if ( 50 < $page->menu_order )
            unset($pages[$key]);
    }
    return $pages;
}

Now before calling wp_list_pages() apply a filter as follows

add_filter('get_pages', 'wpse58346_wp_list_pages', '', 2);

And after you have called wp_list_pages() you can remove the filter so that it doesn’t mess with some other funcctionality

remove_filter('get_pages', 'wpse58346_wp_list_pages');