I've been averaging about $19 per 4tb SAS drive on ebay (refurbs), though sata tends to cost a little more being the more common format.

I've had great luck with surplus HGST drives, they tend to be more reliable than Seagate.

That's very dependent on Amazon actually shipping you the drive, which between my recent experience and tales from a few friends in the industry, is much more of a coin toss these days.