Costco sells 3TB seagate drives (USB3) for ~$120. You can rip apart the enclosure to get a normal 3.5 SATA drive (5900 rpm), and the reliability is good. I've purchased about 10 or so, and they're all still working fine despite 24/7 use.
I have no idea how reliable they are if you leave them in the enclosures. They have rubber isolation mounts, but I wouldn't trust an external drive for anything important.
Edit: I've also tried re-using the enclosures with other drives (both other manufacturers and capacities), and it works fine. You can use the adapter as a dock, but the fit isn't perfect: good enough for temporary use. If installed in the full enclosure everything fits fine.