A hard link just creates a new directory entry for the same file (http://en.wikipedia.org/wiki/Hard_link). So a snapshot takes up effectively zero space, but you'll want to clean it up after you're done copying it off to whatever your archive is, because when the "original" sstable is deleted (typically post-compaction), space won't be reclaimed as long as the snapshot reference is still there.
My impression is that tablesnap is the most popular tool for automating backups to s3. It also supports Cassandra incremental backups. If you want more control over where you're backing up to, DataStax OpsCenter supports running a custom script when it takes snapshots.