I believe it's about .026/GB-month so about $20-30/TB-month. My ultimate goal is to get some scripts going to push the least-accessed datasets into nearline which save me about, say, 30% on total cost.

I consider the total price I pay for cloud backups as a form of punishment for how poorly I manage my data ;) ZFS will let you go crazy and expand forever transparently but you should actually know what data you really need to back up...