I didn't know anything about the NetApp hardware until I saw your post. The DS4243 only supports drives up to 3TB and uses the SATA II interface so it doesn't look like such a great deal when using newer hard drives. I assume the controllers interface with a main server configuration via an SAS interface, but I'm still learning more about it. I didn't really research the controllers at this point so I'm not sure how that works. It looks like the DS4246 is better suited for server applications with larger hard drives. I'm not surprised that you can get the DS4243 so cheap because it's basically obsolete. It's not even listed on the NetApp website anymore. I suppose it's fine if you have a lot of old drives sitting around that you'd like to repurpose.Just picked up a NetApp DS4243 DAS Shelve for $90 shipped.
They won't last long but keep checking back from the seller for more listings.
I'm running an unRAID server with a 24-bay 4U chassis and an 8-bay 2U chassis I believe I posted pics in a recent post). The server currently has 142 TB of storage via 28 data drives, 2 parity drives, and a SSD cache drive. I can eventually expand it up to 224 TB if I so choose, but I have no plans to do that anytime soon. I generally replace the smaller drives as they either fail or start to show signs of failing with 8TB drives. I still have a few 3 TB drives and numerous 4 TB drives in the array. All of them are SATA III drives.