Tarsnap is priced as a utility: You pay for what you use. Use twice as much storage and bandwidth? You'll pay twice as much. Use only 10 MB of bandwidth and 20 MB-months of storage? You'll pay less than one cent. Since making money from rounding errors doesn't seem fair, Tarsnap's accounting is performed in attodollars ($10-18) and rounded in the user's favour.
Most physical utilities, like electricity, natural gas, and water, have fixed monthly fees for "being hooked up to the grid" in addition to their usage-based fees. Tarsnap doesn't — because, let's face it, there is no physical grid to be hooked up to. Having accounts sit around idle doesn't add any significant cost to keeping the service running — so why should you pay for it either?
Similarly, many online services offer a selection of fixed-price monthly "plans" to choose from; these have two major disadvantages. First, they force you to predict how much usage you'll have ahead of time; and second, unless you're incredibly lucky, you'll always be paying for some capacity which you're not using. This second issue is particularly pronounced for small users, who often use only a very small fraction of the quotas permitted under typical monthly plans.
Thanks to Tarsnap's linear pricing scheme and its lack of fixed monthly fees, over half of Tarsnap users spend under $1 per month on storing their backups. This would be economically unfeasible if every user's credit card was billed each month; instead, Tarsnap works on a prepaid model (like prepaid mobile phones). Before you can start using Tarsnap, you need to deposit $5 or more into your Tarsnap account; and then you can add more money whenever you want. Just like a mobile phone, if your account balance falls below zero, your account will be disabled; if your account balance stays below zero for more than a week, your account will be permanently removed.
We have collected a few examples of deduplication efficiency, which illustrates how much money our linear pricing can save.