"Back in the day" I used to spec disk storage (e.g. EMC Symmetrix) for customers who paid around $1000/GB. Then I spent about 2 years away from the system sizing game. When I got back into the flow in Feb 2002 I looked around to figure out new rules of thumb for storage costs. On a continuum from cheap to pricey:

Here's a much more comprehensive look at this subject: http://www.mkomo.com/cost-per-gigabyte

Here's an interesting article about how the pros do large-scale cost-effective disk storage: http://blog.backblaze.com/2009/09/01/petabytes-on-a-budget-how-to-build-cheap-cloud-storage/

2004

type	amount	cost	$/GB	source
Raw IDE disk (Maxtor)	120GB	$120	1	Compusa
SCSI Hotplug	146GB	$500	3.5	Dell
mid-range RAID (Sun T3)	1321GB	$98k	74	Sun Store
high-end RAID (Sun 9970)	9TB	$677k	73.5	Sun Store

Now high-end RAID is about 75 times more expensive than consumer-grade IDE (the difference used to be 25x). It's getting harder and harder to justify going with the big stuff, unless you really really need it.

2002

type	amount	cost	$/GB	source
Raw IDE disk (WD)	20GB	$100	5	Staples
low-end RAID (compaq 4100)	432GB	$15k	36	Compaq website
SCSI "shoebox" (Sun Multipack)	218GB	$5650	26	Sun Store
high-end RAID (Sun 9960)	5800GB	$730k	125	Sun Store
mid-range RAID (Sun T3)	655GB	$84k	128	Sun Store