Using BitTorrent makes it even more cheaper, if more people need to download the file since you only have to pay for the traffic that the S3 seeder generates.(dev docs)

S3 supports the BitTorrent protocol so that developers can save costs when distributing content at high scale. There is no extra charge for use of BitTorrent with S3. Data transfer via the BitTorrent protocol is metered at the same rate as client/server delivery. To be precise, whenever a downloading BitTorrent client requests a “piece” of an object from the S3 “seeder,” charges accrue just as if an anonymous request for that piece had been made

This is how it works:

Any object in S3 that can be read anonymously can also be downloaded via BitTorrent. Retrieving a .torrent file for any publicly available object is easy. Simply add a “?torrent” query string parameter at the end of the REST GET request for the object. No authentication is required. Once you have a BitTorrent client installed, downloading an object using BitTorrent download may be as easy as opening this URL in your web browser.

So if you’re looking for relatively cheap online storage and distribution, S3 if definitely worth trying.

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Source: TorrentFreak, for the latest info on copyright, file-sharing and anonymous VPN services.