The cost of producing any item is directly connected with the volume of unit's it will sell in a feedback loop.
In the case of a conventional hard disk, with spinning rusty glass and electronics, there's potentially a huge variation in the cost of the mechanical and electronic components - however 2 clear price/quality bands have emerged - Enterprise and Commodity.
However the reduced sales volume of an enterprise drive takes a heavy toll on what you get for your money - something which costs seven times as much won't be seven times better.
The enterprise units (for a given capacity) are slightly faster than the commodity units, e.g. comparing Seagate Barracuda SATA (commodity) and Cheetah SAS (enterprise) drives:
model Barracuda Cheetah
capacity(Gb) 500 450
sustained data 125 168
cache(Mb) 16 64
annual failure <1% 0.44%
Price GBP 42 275
Price USD 67 440
But in an enterprise context, no sane system administrator would ever store important data on a single drive - using multiple drives provides greater reliability and bandwidth, and effectively reduces latency; four of the Barracuda drives configured as RAID10, will be a lot faster than the single Cheetah drive, with much less risk of data loss at around 60% of the price.
Certainly you'll get a better warranty with the Enterprise drive, and the vendor will usually be able to get one to you the same day - but you'll probably be able to source a commodity drive from a local supplier faster than your vendor can courier out the replacement disk. On the other hand, the enterprise disk is more likely to be an exact replacement for the failed drive.
So maybe you get a lot more reliability from the enterprise drives? While the people making and selling the drives often say this is the case (Seagate are rather coy about this on their website - but even the obfuscated numbers they publish show it's less than a factor of 2). Independent studies suggest that there's no significant difference.
The SCSI command set does have some technical advantages over the ATA command set - particularly in terms of allowing the OS to know exactly what is committed to the disk - however again, this only makes an effective difference looking at the performance and reliability of a filesystem implemented on a single disk.
If your service provider operates a Fibre Channel SAN, then the cost per Gigabyte of storage will be at least 8 times higher than buying a disk off the shelf at your nearest hardware store. But there are other approaches which can bring the cost down significantly.
Note that this will still never be cheaper than buying an off-the-shelf disk, as you are also paying for redundancy, power, air-conditioning and support, but these costs should be small in relation to the cost of the storage provision.