It's true that Glacier is a bit tricky, but you can easily cap your retrieval costs (with a bit of margin) if your use-case allows you to choose a maximum retrieval rate, rather than thinking in terms of retrieval amount. If your peak hourly retrieval rate for a month is N Mbps, your cost is no more than $3N. So for example, $30 for a month that includes 10 Mbps retrievals, $300 for 100 Mbps retrievals, etc.

A bit more detail in an older comment of mine: https://news.ycombinator.com/item?id=7147325