We're talking about a difference of $10k USD
per instance per year.
You'll want at least two for redundancy so that's $20k spare change if you go dedicated. That buys quite lot of rackspace, network IO and spare parts.
Staff salaries don't factor in because if you need storage of that scale you can't do without a competent admin either way (the $20k comfortably pay for a fully managed colo with remote hands).
Needless to say most deployments of that size will need more than two boxes, at which point the markup tips entirely into wtf-territory. Note my calculation was really generous here, too. In reality you get steep hardware discounts on top that make amazon look even worse, and you can buy boxes with higher storage density for an even better $/GB.
