The drives themselves will usually be high-end, high-rotation drives, possibly with custom firmwares installed to run on the particular SAN. They are probably sole- or restricted-sourced, meaning the SAN vendor is paying a bit of a premium to ensure that they can get EXACTLY these drives for the next period of time.
There's far more to a SAN than just hard drives.
There's the RAID controllers. There's the networking components. There's the physical shell the drives come in, with associated power (frequently redundant). There's the software that has been written to run, monitor, and manage the whole thing. There's a large quality and testing organization which is there to ensure that when you get it, it "just works". And there's probably a warranty service that will have a replacement drive show up within 8 to 24 hours should one fail.
Plus there's the fact that you are using this to gain some cost- or operational-efficiencies, and the vendor is simply monetizing some of that efficiency for themselves.
Sure, you could buy 12TB of drive space for $1500. But you would still need a computer (or two) to put them in, an OS to present them to the network, and someone to make sure it would work and be reasonably redundant.