Do you want to use it as a purely backup type thing or as the place that holds your only copy? If it's more a backup type thing, something like backblaze or crashplan at $5/month/computer gives you unlimited backup storage. Crashplan also has a 'family' plan that is $12/month for 10 computers.
Crashplan also lets you 'seed' a backup with a 1TB HDD that they ship you for $125.
At the risk of being pendantic, I don't think there should ever be an only copy of anything remotely important. (and even 1 local + 1 remote controlled by a single 3rd party is probably not a good policy if it is significant)
Crashplan also lets you 'seed' a backup with a 1TB HDD that they ship you for $125.