It's definitely possible to offer that on a monthly basis if you model that each customer stays for 36-39 months. Also, I doubt that they are using replicated storage, but are using erasure coding instead. Also, they dedupe before upload, so more cost savings there.
Spideroak can and does dedupe client side before uploading. It can't dedupe across multiple clients, but it does dedupe within the client. It also tracks syncs so that data synced between multiple client machines only has to be stored once (with appropriate redundancy).
