I can't see how they can be using Amazon storage. I don't believe I am a heavy CrashPlan user with 1.2TB backed up (all most all from my active machines).
At Amazon's cheapest published rates it would cost $33 a month for standard storage and half that for infrequent access storage (I don't believe with the way CrashPlan operates that they could use Glacier). If I remember correctly I'm paying them around $10 a month.
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Remind me to change my signature to something more interesting someday