Add this to your functions.php in your template folder.
<?php
//Set Default Meta Value
function set_default_meta_new_post($post_ID){
$current_field_value = get_post_meta($post_ID,'YOURMETAKEY',true); //change YOUMETAKEY to a default
$default_meta="100"; //set default value
if ($current_field_value == '' && !wp_is_post_revision($post_ID)){
add_post_meta($post_ID,'YOURMETAKEY',$default_meta,true);
}
return $post_ID;
}
add_action('wp_insert_post','set_default_meta_new_post');
?>