Respectfully, Colin, but you price Tarsnap horribly. You run it like a public utility (a very complicated public utility) instead of a business.
You think this is a good thing, but it's not. Instead of setting a simple price based on what Tarsnap is worth to people --- a task that requires figuring out what Tarsnap is worth to different people and selecting the segment of the market that(a) you want to serve and (b) is sufficiently lucrative --- you have a fundamentally cost-based micro-metering structure that almost certainly radically undercharges most of your users.
You may think you're doing your users a favor by letting them off easy. But you're not. If Tarsnap is worth $11/mo to most of your users (hint: it is) and you charge $10/mo, you're generating value for them. But instead of doing that, you charge them almost nothing, and as a result are much more limited in what you can do for them. Last I checked, you were consulting instead of full-timing Tarsnap.
If you charged more, you could afford to:
* Radically overhaul your website and educate users about what secure backup actually means
* Commission a better user interface for people who you'd like to offer secure backup to but can't because they're not going to use a Unix command-line tool
* Implement more and better features on the backend of your system for tracking data, scheduling backups, backing up different kinds of data, and whatnot
This is all apart from the obvious point that your micro-metered pricing scheme is a Mensa test that almost makes fun of people's attempts to figure out what your service costs. Yes, the fact that it costs much too little means the joke is on you, but that doesn't really take the sting out of it for your prospective customers.