The price is quite good if you calculate for yourself the cost of 5 drives to get to 20 TB, cost of a big enough enclosure with cooling, SATA controllers and a power supply you end up not far from what they're asking. Of course their solution is a bit more expensive but you get a beautiful, compact, small yet high efficiency enclosure with hot swap capability and Thunderbolt connectivity which wasn't included in my comparison.
I could not build a box that size for the same price. I tried. Ended up with a mid-tower unit. Yes, it holds 8 instead of 5, but it's also a lot bigger than could be.