Each vendor will have different prices. It will also depend on the size of the disks. For 73 Gig spindles the cost per Gig will be higher than if you purchase 1TB spindles.
For example the EMC storage that I just purcahsed:
FC 173 Gig spindles ~ $6 / Gig
FC 400 Gig spindles ~ $3 / Gig
SATA 1 TB spindles ~ $1.50 / Gig
It will also depend on how large your order. Larger orders will get you a better discount than smaller orders will.