Just use a custom field. On post or page edit page click Screen Options in the top right corner anb select Custom Fields. Add a Name and Value pair for example: price (for the name) and 9$ (for the value).
Then in your single.php or whatever template use this code to display it:
<?php echo "Price:" . get_post_meta($post->ID, "price", true); ?>
Reference:
get_post_meta