Copy price categories to custom field

paste this code in you theme’s function.php file

//get all categorie ID's in to array
$args = array( 'child_of'  => 3678); // Parent category ID
$categories = get_categories( $args );
$cat_array = array();
foreach ($categories as $category) {
    $cat_array[] = $category->term_id;
  }

$Query_args = array(
    'category__in' => $cat_array,
    'post_type' => 'YOUR_POST_TYPE_NAME'
);
$new_query = new WP_Query();
$new_query->query($Query_args);
if ($new_query->have_posts()){
    while ($new_query->have_posts()){
        $new_query->the_post();
        foreach((get_the_category()) as $category) { 
            if (in_array($category->cat_ID,$cat_array)){
                update_post_meta($post_id, 'price', $category->cat_name);
            }
        }
    }
}

replace “Parent category ID” with price category ID and then save that file
after that remove this code and save again.