No offense, but this post doesn't quite match what you wrote originally. My sigh was in response to the phrase "Dropbox should work here, ...". You didn't state security as your concern as to why not use Dropbox, rather it was cost. This might lead someone who only needs <2GB backed up to believe that Dropbox is perfectly fine for that task.
"DropBox is used for backup because they've made file sync so damn easy that most people can be convinced that if a file exists in many places, it is backed-up."
And that's exactly what I'm scared about and why I'm saying it again and again that you shouldn't do it - if only one person listens and avoids potential data loss because of it I've already reached my goal.
"One of the things about backup is that it needs to be easy. Currently the size and cost of backups make it expensive, and the only way to reduce the cost makes it difficult (HDD local copies stored at a friends' house for example). By reducing the cost, perhaps we can finally increase the ease... and then a day may come in which most people have a real backup solution."
Agreed. IMO no consumer backup system is quite there yet. TimeMachine is very close, if only it would do better logging and have some more intelligence about warning messages.