In order to give you a zpool of your own we need to give you a full blown VM (bhyve) with resource guarantees and your own IP address, etc.
So there is a 1TB minimum and no discounts.
Alternatively, if you just get a plain old rsync.net account the minimum account size is much smaller and if you're an expert and don't need any (borg specific) support there is a discounted plan[1].
If OpenZFS supported some kind of "virtual zpool" feature (I guess it doesn't exist yet) that you could provision out of your main ZFS infrastructure and hand off to the customer, would that be useful to reduce the resource overhead related to offering ZFS directly?
Do you have a sense of why dedicated/isolated infra (VM & IP) might be necessary to provide this service?
We strongly encourage people to use ipv6, which we support everywhere ...