I think it's cheaper to buy a whole JBOD chassis plus the necessary cables, HBA, and smaller drives than to use the latest, greatest supersized drives in your current chassis. Even with this great deal, it's a 12TB drive at $6.67/TB, and I just bought a bunch of 8TB SAS drives for $5.13/TB. I only need 1.5x as many bays for the same total storage. That's another reason the really big drives are so expensive...there are a lot of people with only a 4-bay NAS that want 60TB of storage, so they have to buy 20TB drives.
As for SAS, I have generally found that once you get larger than about 8TB, SAS drives are more expensive on the used market than enterprise SATA. I suspect this is because SAS are still much more expensive when new, and larger drives are newer, thus the cost hasn't been amortized long enough to bring the price down on the used market.
For spinning rust, actual performance should be the same on SAS and SATA, even though SATA is limited to 6Gbps and you pay a slight penalty for tunneling SATA over an SAS connection.