A lot of people do not do backups at home. Unfortunately this includes me. So far I was lucky that nothing bad happened (or that I was able to get back all via manipulating on-disk-data by hand). For me this is mostly because of the price and complexity (number of backup media involved) for local backups. It also means that I only need a backup for the case of a disk–crash , not because I need to recover a file because I accidently deleted it (yes, I am very good at not loosing data at home… maybe this is also the reason  why I have so much data).
I have about 1 TB of raw disk space at home (two times raidz1). Not all of this needs a backup (or is used at all), for example the base system files, /usr/local, the ports  distfiles and packages all do not need a backup, but my camera fotos (currently 8 GB) should be included in a backup, my home should be included in a backup, my local staging area for the content of my webserver should be included in a backup, my mails (currently 800 MB) should be included in the backup, local patches to FreeBSD  should be included in the backup, …
So let us calculate with about 200 GB of more (mails, fotos, private videos) or less (MP3s from my own CDs) important data for a full backup. Most of it will not change often (MP3s, private videos, fotos), so once a month should be ok, and some (e.g. my mails) change daily, so an incremental each day and a full once a week would be interesting for me. When I do such a backup, I do not want to shuffle around tapes a lot. Maybe one or two times (s0 2 – 3 tapes) would be ok. So I am talking about 80 – 200 GB per tape. The prices for this range from 700 EUR to 1300 EUR just for the tape drive. I have also seen a tape drive from Iomega  for about 400 EUR (120 GB per tape), but somehow this sounds like a not so trustworty solution to me (I do not know why, it is just a feeling, maybe I am a little bit biased because of the high-end solution at work, if someone knows more about those Iomega  drives and tapes in a long term or high-useage environment, please write a comment).
When I take the current price of tarsnap  into account, I come to about $60 per month for the storage, and again $60 for the initial transfer of 200 GB. This assumes the 200 GB are not very compressable. With each incremental backup (changed files matter, not a diff between the files), I assume about 10 GB per month of change (= $3 per month). Yes, this is most probably too much, but I try to calculate a worst case scenario. So this sums up to $60 once and $63 per month. I do not take into account my too slow upstream bandwith. When I assume a 1500 EUR tape drive plus tapes plus cleaning cartridge, we are talking about something like 2 – 3 years of storing the backup in tarsnap (with the Iomega drive this would be just one year).
When I assume that I overestimated everything with a factor of 2, this means 5 – 6 years of using tarsnap vs. buying an expensive tape drive. Now the question is, will the tape drive survive this long, will I be able to use such a drive in new hardware  in 6 years, will tarsnap survive this long, and will it stay at the same or better price level (this is also influenced by the price of the storage provider).
Another solution would be to go with a mixed setting, e.g. an 1 TB hard-disk (less than 150 EUR for the hard-disk (with 7 years warranty) and an external case) to do the long term storage of high-volume but long-term–stable  stuff  (fotos, videos, music), and use tarsnap for the low-volume fast changing stuff (config files, mails). This way the amount of money  for the really important things is not much, I would expect less than 1 GB compressed (= less than an EUR per month). This also sounds like a solution which allows me to be lazy (the important stuff can be automated, the nice to have stuff can be done from time to time as needed).