If you check the FAQ, they bill you based on your peak hourly retrieval rate.
If I download 4TB at say...10MB/s, not only do I need to pay $480, but I also have to pay ~$257 as a retrieval fee?
Their wording is confusing, but ignoring the free retrieval amount (negligible difference on a 4TB transfer):
Fee = Peak hourly retrieval * number of hours in month * $0.01
https://aws.amazon.com/glacier/faqs/#How_much_data_can_I_ret...
Admittedly ~$737 isn't the end of the world if your house has burned down and you need all your data back, but it's still important to know the details.
I think in that situation, it would be cheaper to use their bulk import/export, which would be roughly $300 for 4TB
Edit: So I'm working through a scenario in my head and trying to figure out how charging based on the peak hour isn't completely ridiculous.
I have 8GB stored to try out the system. This costs a whopping dollar per year. One day I decide to test out the restore feature. So I go tell Amazon to get my files and wait a few hours. When Amazon is ready, I hit download. I'm on a relatively fast cable connection so the download finishes in an hour. I look at the data transfer prices and expect to be charged one dollar.
But I didn't take into account this 'peak hour' method. I just used roughly 8GB/hour over the minimal free retrieval. This gets multiplied out times 24 hours and 30 days to cost 8 * 720 * $0.01 = $57. Fifty-seven times my annual budget because I downloaded my data too quickly after waiting hours for Amazon to get ready.
Realistically though, this service might not be for you if fast and cheap retrieval of your data is important. The importance here is cheap storage, not transfer. They could reasonably expect that you'd only retrieve this data once or twice, if ever, and cost won't be deterrent. Say, if your data center burns down and your company is moving to a new office.
But ultimately, this product isn't designed for backup purposes. It's designed for archive purposes. If you have 4TB of customer data from 3+ years ago that you never access, but need to keep in case the IRS does an audit, then this is the place to put it.
