That's kind of another problem though. No way to predict costs in advance. All you can do is know the upper bound.

I have approx 2.5tb on CrashPlan, so that gives me an upper bound of $625 per month. Maybe it might be 50% lower when deduped, maybe more, maybe less. Who knows. Not really a pricing model I can make a decision on.

Tarsnap looks great for certain use cases. But it doesn't really make sense for home users with a large amount of data.