Really, do you or your clients know?
Would you be able to provide a valid and true number, or at least a reasonable estimate?
Is Google/Microsoft/IBM or "Fred's Cloud" really a cheaper more efficient solution?
On what do you base this?
Ed Brill's post had some comments which I was going to reply, but instead wanted to expand my answer.
What is really being saved is the question of course.
No company stops needing their telecom, wiring, firewall, routers etc just because they go to the Cloud. In fact one could argue some of these INCREASE.
Nor do they stop needing their administrators or help desk or support staff. Although you may be able to drop a few people or move them to a different project, they can't all be user/password admins.
Possibly the costs of some hardware(the onsite servers), SAN/Drive space and some related items, plus NOS cost and licenses, and their support/maintenance contracts are what is saved by going to the cloud.
What other costs are there you ask that could get saved? Backup solutions and costs, archiving, spam filters, antivirus which all have fees or maintenance.
Need more? Electricity, Real Estate/Data Center Space, A/C requirements.
Still need more reasons? Leave some in the comments.
What am I missing or not thinking of right now?
I will put together an ROI for it all once I have inputs from everyone.
There is no price for headaches, they are priceless. Going to the cloud may limit the ones you feel today, but will create other ones potentially even greater or worse. (Worse would be when Fred's Cloud goes under)