I've spec'ed out S3 alternatives for clients, and pretty much no matter what criteria they have for redundancy, managed hosting solutions or co-location comes out much cheaper than S3. E.g. typically half or less of the S3 costs for triply redundant servers, on RAID arrays, in separate data-centres with replication (e.g. using OpenStack Swift, or Gluster / Ceph + an API layer for example)
For larger setups you can do it at 1/3 or less of the cost if you're prepared to go the colo route (rather than managed servers) and leasing your own hardware.
If your average bandwidth usage per object is high you can trivially cut costs far beyond that, as the EC2/S3 bandwidth costs are atrocious.
The exception would be if you can't avoid huge amount of object accesses from within EC2 even if you move the storage out of S3.
For the vast majority of people, S3 means cheap, mass storage.
For the majority of those people, a simple SFTP/rsync/NFS/whatever endpoint (potentially with replication to provide redundancy) would more than fit the bill, and would actually be simpler to use than the S3 API.
I store about a terabyte on S3. It costs us $25 a month. If I spend more than a few minutes a month administering an alternative setup, I've lost money compared to just paying S3.
That entirely depends on how you use the data. If you store files that are accessed over the general internet, and each file is downloaded just once in the month, you've just quadrupled your monthly cost, thanks to AWS' high data transfer fees.
Running an FTP server != an S3 equivalent.