Glacier bills on "peak retrieval rate". Say you have a 100mbps connection and 1TB of data. Retrieving all 1TB over a 24-hour period at an even rate will cost you ~$300. Not great, but not awful.
But this calculation is wrong. The real story is much worse.
You're not retrieving your data directly from Glacier. You're restoring it to S3, then downloading your files from there.
Amazon is going to bill you at the peak retrieval rate _from Glacier to S3_. If you're lucky and it takes four hours (and at an even rate), you'll rack up _$2000_ in charges.
This has happened to me and to many others. The solution is to carefully meter your restoration requests. Or, better yet, don't use Glacier in the first place. It's simply not suited for the everyday user.
Play with the numbers for yourself: http://calculator.s3.amazonaws.com/index.html
And I'm certainly not the first to warn of this: