What gives? I can get a stack of 1TB drives for about a grand, and even my personal computer at home has a rudimentary RAID. Is all NAS that expensive,
I run into this exact question quite a lot internally when we tell users how much it costs to add more space to their allocation. It is eye-bleedingly expensive on one of our arrays, and this is a major, major problem. However, it is a forklift upgrade (6 digit replacement cost) so won't happen in the next 12 months. This is a 'miser' failure-mode.
I did run the cost-per-GB numbers on a pair of our arrays on a blog post of mine from two years ago (link if you want to read the whole thing). On one array, the cost-per-GB was a whopping $16.22, on another array it was $3.03. Once you factor in backup and disaster recovery coverage costs into the deal the numbers rose to $26 and $7.50 respectively. This is for Fibre Channel attached enterprise storage designed to service a lot of I/O requests at the same time.
I just finished writing the spec for another new array system that is targeted at a cost-per-GB of about $2/GB (not including backup/DR coverage). This is for an iSCSI/NAS device. The lack of fibre-channel makes it significantly cheaper, but we pay for it in the form of less resilience in the actual array. It can pump out the I/O operations, but it simply won't scale as far as the FC-based ones.
When figuring out the cost of storage per GB, you have to keep in mind that data is duplicated any number of times. RAID level impacts this. How deep your Backup infrastructure runs is a significant cost-factor (if you have to keep 7 years of data, and you keep one full copy each week, your data is replicated potentially 365 times. That's a lot of tape, and is why Deduplication vendors charge what they do). The storage infrastructure directly supporting the actual hard drives needs to be amortized across all of the storage contained in that infrastructure. Plus any support contracts needed to make it all supportable.
What further adds to the misery of big-storage needing departments is that the storage vendors are the LAST to roll out the big drives. Those 2TB SATA drives you've been seeing for quite a while now are only now beginning to roll out from the likes of HP, EMC, and Dell. This is due in large part to those brand new storage tiers not having enough usage history behind them to bet the farm on (or put another way, accurately price the support contract costs). That cheap array I recently built is based on 750GB 3Gb SAS drives, not 2TB SATA drives.