YOU look again.
"The pricing for PM863s is slightly under linear (double capacity slightly less than double price) and a 4TB PM863 is about 3 times the price of an _enterprise certified_ Hitachi 7k 4Tb drive (which are about 400 quid for my Nexsan Satabeasts, not 100 and some change)"
So 1200 each. In case you haven't noticed, that's a pretty hefty chunk of change for 4TB, and your mileage may vary in regards to longevity. We've already heard plenty of stories of sudden catastrophic controller failure.
Plus you didn't note my last sentence:
"and there's no analogue to them at the consumer end."
I just recently bought a pair of 5TB USB externals for $350 (two to provide a mirror) and a 4TB SATA internal for $150. Please show me a 4TB SSD for less then $750.