Amazon offers the Western Digital Red IntelliPower 5TB Red Serial ATA 6Gb/s 3.5" Internal Hard Drive, model no. WD50EFRX, for $168.99 with free shipping. That's $11 under our May mention and the lowest total price we've seen. (It's also a current price low by $26.) It's optimized for use in NAS systems and features a 64MB cache.
Note: It's temporarily out of stock, but it can be ordered at this price for shipping when available.