Wow, where do you come up with $8.6/GB?
Hitachi's 300GB SAS disks are about $350, so that's $1.2/GB raw.
If you are talking usable space, say you have 10 disks per shelf combined in RAID 6, that's 7 usable disks, 2 for data parity, 1 as hot spare. Add another shelf for backup, that's still only $3.3/GB with RAID redundancy, backup and hot spares (probably removing another $19.5/GB from the lines for backup disk, DR backup disk, DR disk, DR backup disk, and the tapes, that really don't scale for big datacenter. $3500/TB tape "software"?
SATA III enterprise-level disks have the same 64MB cache as SAS disks and will probably have about the same IOPS once SATA III RAID controllers come along, plus controllers already use SSD caching for even better performance, so the cost of the same configuration as above is only $0.2/GB, for 14TB of usable space in 1 shelf, compared to 2.1TB with the Hitachi SAS disks.
Now that SSD is starting to mature, I think SAS SANs are doomed. SAS will never reach the same performance and reliability as SSD (no mechanical parts) for the same price. Even now, SSD raw disk cost is $1.4/GB ($700/512GB) compared to SAS $1.2/GB.