I am impressed. Yes, really. It seems tarsnap DTRT.
I made a test with tarsnap. I made a backup of some data (a full backup of everything is kept on a ZFS volume on an external disk which is only attached to make a full backup once in a while) of one of my systems. This data is 1.1 GB in size (most of it is /usr/src checked out via subversion and extended with some patches – no music, pictures or other such data). This compresses down to 325 MB. Of this 325 MB of data only 242 MB is stored encrypted on the tarsnap server (automatic de-duplication on the backup client). The second backup of the same data in the following night (again 1.1 GB in total, 325 MB compressed) caused 216 kB of new data to be stored on the tarsnap server (again, de-duplication on the client). What I have now are two full off-site backups of this data (two archives with 1.1 GB of data after decompression), with the benefit that the additional storage space required is the storage space of an incremental backup.
The cost (in dollar) of this so far is 0.074603634 for the initial transfer of the data, 0.00242067783 for the data storage on the first day, plus 0.0019572486 for the transfer for the second backup. From the initial 29.93 I still have 29.85 (rounded) left. If I factor out the initial transfer and assuming that the rate of change for this system stays constant, this comes down to 0.01 (rounded-up) per day for this system (or about 8 years of backup if I do not add more systems and do not add more than the initial 29.93 (= EUR 20) – and the price of this service does not increase, off course). Is this data worth 1 cent per day for me? Yes, for sure! Even more, but hey, you did not read this here.
That is what you get when a person designs a service which he is willing to use himself for a price he wants to pay himself (while still not lose money with the service).
Colin, again, I am impressed. Big thumbs up for you!
P.S.: Yes there are also places to improve, I found already some things (the config file parser is a little bit strict what it accepts, and some things should be more documented) but Colin is responsive and open to improvement suggestions.