For large data centres a big industry player estimated costs of £22 / GB / Month = £250k / TB / Year. Majority of this was hardware and energy costs (not costs of human sysadmins). This seems quite a lot. However, Amazon S3 quote for Europe (cheaper for US):
Storage
$0.18 per GB-Month of storage used
Data Transfer
$0.100 per GB - all data transfer in
$0.170 per GB - first 10 TB / month data transfer out
$0.130 per GB - next 40 TB / month data transfer out
$0.110 per GB - next 100 TB / month data transfer out
$0.100 per GB - data transfer out / month over 150 TB
Requests
$0.012 per 1,000 PUT, POST, or LIST requests
$0.012 per 10,000 GET and all other requests*
* No charge for delete requests
Subtracting say £2 for costs of storage and transfer leaves £20 per GB Month = $40 / GBM. On Amazon’s figures this is around 235 GB of transfer (0.235 TB). A ratio of 235 to 1 on the underlying data. Not necessarily an infeasible level (235 users / byte / month). This also demonstrates that b/w costs will dwarf storage costs in most cases.