The real way to get cost effective storage is not buying more servers, licenses, and cheaper disk drives.
It's to go with a solution that work across multiple apps. It's virtualizing Exchange with Hyper-V or VMware.
It's allocating DB space with thin provisioning and SATA drives. When you price out SAN with thin and SATA, it's not so bad. If you use little 15K FC drives, of course that looks expensive!!
Even if the cost per GB is a bit cheaper with DAS, you end up spending so much more to manage it. and to cool it. And then, if you decide to try a hosted model, you have tons and tons of DAS sitting around, where if you have a SAN at least you could re-use the space pretty easily.
Just some common sense.
B