Adding meta data to an attachment post

So since attachments are post types, you should be able to assign them postmeta like any other post type.

Using the update_post_meta function should get you where you need to be.

update_post_meta( $attachment_id, 'price', $price );

http://codex.wordpress.org/Function_Reference/update_post_meta

Then run your query based on a meta key.

$args = array(
'order'          => 'ASC',

'post_type'      => 'attachment',
'post_parent'    => $post->ID,
'post_mime_type' => 'image',
'post_status'    => null,
'numberposts'    => -1,

'meta_key'       => 'price',
);

http://codex.wordpress.org/Template_Tags/get_posts#Parameters