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Can I sort by custom fields?

Yes and no. In order to sort by custom fields, you also need to use the posts_join filter in addition to the posts_orderby filter.

For the posts_orderby filter you can use the plugin, but for the posts_join filter, you're going to have to write the code yourself.

Just add the following code either in your theme's functions.php or in a new little plugin:

add_filter('posts_join', 'my_posts_join');

function my_posts_join($join){
    global $wpdb;
    $join .= " LEFT JOIN $wpdb->postmeta wpmeta ON ($wpdb->posts.ID = wpmeta.post_id AND wpmeta.meta_key = 'price')";
    return $join;
}

Replace 'price' with whatever your custom field is called.

Now you have access to your custom field by using wpmeta.meta_value as your Sort Type.

Of course, if you need the numerical value of your custom field, you can use something like ABS(wpmeta.meta_value).