At present Updates cannot be sent for products using a Remote Product File URL, rather than an uploaded file, but even if it were possible, the cost would be the same, as the Update fee covers the additional file-transfer bandwidth costs to provide a sudden surge of downloads for the file, rather than mere file-storage costs.. Even when the original copy of a file is hosted remotely, the downloads are still provided through our network from a cached copy of the file on our server; this is how we cloak your remote file URLs and enforce your link-expiration settings.
You might consider just parking a copy of the updated file on an affordable file-hosting service such as Amazon S3, then send a Newsletter (rather than an Update) with a link directly to that file. This would bypass our download delivery service, so you wouldn't be able to issue each recipient a unique, expirable link, but it would be far more affordable than sending an Update. You could just delete that file after a few days to minimize the potential for unauthorized downloads, once you're sure most Newsletter recipients who'd want the free update have obtained their copy.