But that tape isn't good for 20 years. And you're leaving off the cost of having Iron Mountain periodically bringing those tapes back, someone sticking them in tape drives, verifying the integrity of the backups, and shipping them back to Iron Mountain. At that point Glacier is dirt cheap at $0.01/GB.
Also, inbound data to Amazon is always free. Your upstream to them is free as well, as you're already paying for an Internet connection. Don't want to use your internet connection? Ship 8 4TB drives to Amazon and they'll load the data into Glacier at their facility for you.
If you're not verifying the data periodically (i.e. bitrot), it might as well not exist.
Disclaimer: I store several PBs of data in Glacier.