Save default value for empty or missing post meta

If you want to and only need to do what @tf suggested, what is just to take care of displaying a 0 if no value is present, you can construct a function to do so like this:

function wpse121165_return_carprice() {
    $ecpt_carprice = get_post_meta($post->ID, 'ecpt_carprice', true);
    if(! empty( $ecpt_carprice ) ){
        return $ecpt_carprice;
    } else {
        return 0;
    }
}

Use the function like this: echo wpse121165_return_carprice();

If you really need to update your database, you have to do this another way. Below code should give you an insight on how to do it:

 function wpse121165_update_carprice_meta() {
      // args to query for your key
      $args = array(
        'post_type' => 'your_post_type',
        'meta_query' => array(
            'relation' => 'OR',
            array(
                'key' => 'ecpt_carprice',
                'value' => 'bogus', // you have to pass a value
                'compare' => 'NOT EXISTS'
            ),
            array(
                'key' => 'ecpt_carprice',
                'value' => ''
            )
        ),
        'fields' => 'ids'
      );
      // perform the query to get back an array of ids
      $not_exist_or_empty_ids = new WP_Query( $args );

      foreach ( $not_exist_or_empty_ids as $id ) {
          update_post_meta($id, 'ecpt_carprice', '0');
      }
 }

Use the function like this: wpse121165_update_carprice_meta();. If you put this in you functions.php and perform your meta update on the database, make sure to disable it afterwards and not to call it over and over again.

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