Oh it gets much worse if you assume that you replace your hard drives every 3 years (their warranty period) which would add a $1.94/month depreciation cost.
You could add into the cost / GB of bandwidth to access the files, not as easy to factor into a TCO model in the personal use case.
It gets better, not worse. At steady state you just account for the depreciation cost, though, right? So that's $1.94/TB/MO/Drive, or $0.0019/GB/MO/Drive or $0.01/GB/MO for 3-way redundancy.
The way I was accounting for it you have to replace the drive in 3 years, you have 3 drives. So you've got a cost of $210 that recurs every three years (purchasing the drives). If you distribute that cost across the 36 months that is a $5.83/month fixed cost (doesn't vary by storage usage because you have to replace the entire drive). Unlike Google which can amortize depreciation on a per-GB basis because they are spreading out the replacements amongst many thousands of drives, as an individual you're on the hook for your own drives regardless.
Also, don't forget bandwidth charges if e.g. some of your s3 access does not come from ec2.