Glacier's pricing is actually very difficult to calculate. If you only ever store data (and never retrieve it), it's easy, but the moment you need to start accessing it, the pricing is actually very difficult to calculate correctly:

> Unlike every other service Amazon provides, with Glacier you're not paying for your net usage throughout each month: Instead, you're paying for your peak hour — and once that peak hour has come and gone, you're still paying for that peak until the end of the month, no matter how low your usage might fall. [0]

Note: I still use Glacier for long-term backup, because that's the exact use case it's meant for. But it's important to be aware of that cost: if I ever need to retrieve that data, I know I need to do it very slowly (and carefully) to avoid racking up a massive bill.

[0] http://www.daemonology.net/blog/2012-09-04-thoughts-on-glaci...