The retrieval fee for 3TB could be as high as $22,082 based on my reading of their FAQ [1].
It's not clear to me how they calculate the hourly retrieval rate. Is it based on how fast you download the data once it's available, how much data you request divided by how long it takes them to retrieve it (3.5-4.5 hours), or the size of the archives you request for retrieval in a given hour?
This last case seems most plausible to me [6] -- that the retrieval rate is based solely on the rate of your requests.
In that case, the math would work as follows:
After uploading 3TB (3 * 2^40 bytes) as a single archive, your retrieval allowance would be 153.6 GB/mo (3TB * 5%), or 5.12 GB/day (3TB * 5% / 30). Assuming this one retrieval was the only retrieval of the day, and as it's a single archive you can't break it into smaller pieces, your billable peak hourly retrieval would be 3072 GB - 5.12 GB = 3066.88 GB.
Thus your retrieval fee would be 3066.88 * 720 * .01 = $22081.535 (719x your monthly storage fee).
That would be a wake-up call for someone just doing some testing.
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[1] http://aws.amazon.com/glacier/faqs/#How_will_I_be_charged_wh...
[2] After paying that fee, you might be reminded of S4: http://www.supersimplestorageservice.com/
[3] How do you think this interacts with AWS Export? It seems that AWS Export would maximize your financial pain by making retrieval requests at an extraordinarily fast rate.
[(edit) 4] Once you make a retrieval request the data is only available for 24 hours. So even in the best case, that they charge you based on how long it takes you to download it (and you're careful to throttle accurately), the charge would be $920 ($0.2995/GB) -- that's the lower bound here. Which is better, of course, but I wouldn't rely on it until they clarify how they calculate. My calculations above represent an upper bound ("as high as"). Also note that they charge separately for bandwidth out of AWS ($368.52 in this case).
[(edit) 5] Answering an objection below, I looked at the docs and it doesn't appear that you can make a ranged retrieval request. It appears you have to grab an entire archive at once. You can make a ranged GET request, but that only helps if they charge based on the download rate and not based on the request rate.
[(edit) 6] I think charging this way is more plausible because they incur their cost during the retrieval regardless of whether or how fast you download the result during the 24 hour period it's available to you (retrieval is the dominant expense, not internal network bandwidth). As for the other alternative, charging based on how long it takes them to retrieve it would seem odd as you have no control over that.