http://bioteam.net/2011/08/real-world-backblaze-costs/

Projected cost from Backblaze was $7384, for drives at $120 each and other parts sourced in quantity. A lot of the expense for these guys came from the $5400 everything-but-the-drives kit from Protocase.

The drives were more expensive because a certain model of 3TB Hitachi drives are the parts recommended by Backblaze; they didn't get whatever happened to be on sale at Newegg that day. The drives were supposed to be $120 each in Backblaze's quantities. They ended up finding a sales rep at CDW.com who sold them for $129 in quantity, but they were more expensive elsewhere.

Also: depends what you're considering "pro drives", but enterprise SATA 3TB disks are in the $250 - $300 range -- about 2x the price of a similar consumer-grade disk.

Different brands makes the separation more or less clear : WD desktop drives firmwares are explicitly crippled to be made almost unusable in RAID arrays (on the web and forums you'll find countless horror stories of lost arrays); Seagate and Hitachi desktop drives work about OK in RAID arrays, but you may have surprises at times.

So what's the difference? First, the desktop drive assumes to be alone. In case of a read or write error, it will try and retry to access the data for several long minutes (long timeout). Pro drives assume to be in RAID arrays, in case of an error they fail almost immediately not to block any outstanding IOs to the array.

Another difference is in vibration compensation. Desktop drive don't use their movement detectors to compensate from drive-induced chassis vibration, which in case of high IO will significantly reduce throughput by augmenting error rate.

It sounds like what you're saying makes sense, from a drive business and manufacturing perspective.

This is NDA information :) I'm repeating what I've heard from drive makers and my experience after setting up several thousands of RAID systems. I didn't made tests recently, but vibrations can kill an array performance. I've seen a chassis where the central drive slot (among 24) wasn't usable because it vibrated more than the others :)

This seems a bit off considering Backblaze demonstrates right here that you can care about your data and your budget at the same time.

If you are going to use high-end 'pro' drives then wouldn't you just buy a high-end enterprise grade SAN anyway?

I use that number to figure out how much enterprise storage chassis actually cost by subtracting the number of disks * $320. :)