Just out of curiosity: What is the data volume that gets "fixed" that way? I guess there will be many people out there crying out loud, if the N+1 would mean an additional 100k$ or so. But relative to other prices (esp. some software) that is not really so much money for a larger enterprise and its overall IT budget.
Best practice for ZFS is to maintain occupancy rates someplace between 10%-50% for VM block storage, depending on the amount of churn.
With used X9 2U 12 bay servers going for $500, 512GB DDR3 for $800, 1TB NVMe for $150/ea, and 14TB drives shucked for $200/ea, you can assemble a basic system (2 NVMe L2ARC + 12 14TB disks) for about $5K if you shop frugally. If your storage policy requires redundancy to be maintained in the face of a disk failure, this means three-way mirrors, so four 14TB vdevs are possible, 56TB in the pool, of which 5-25TB-"ish" are usable for block storage. SLOG device and network card not included.
You can, of course, buy new gear at significantly higher prices and get a little bit more performance out of it, but even then, the prices are very reasonable compared to your typical EqualLogic or NetApp.