global $wpdb;
$meta = '_project_price';
$query = $wpdb->prepare(
"SELECT min( cast( meta_value as UNSIGNED ) ) FROM {$wpdb->postmeta} WHERE meta_key='%s'",
$meta
);
$result = $wpdb->get_var( $query );
wp_posts
и в where добавьте wp_posts.post_type != "revision"
SELECT min( cast( meta_value as UNSIGNED ) ) FROM {$wpdb->postmeta} LEFT JOIN {$wpdb->posts} ON {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id WHERE meta_key='%s' and {$wpdb->posts}.post_type != "revision"