OK so I just wanted to add a few more ideas that are worth exploring..

Data Centre Consolidation
The idea of saving money by deploying Exchange Servers in fewer locations is not a new one and something which many companies have explored; but Exchange Server 2007 does change things slightly and so this maybe worth revisiting.

The I\O footprint of an Exchange 2007 Server is lighter and responding to disk requests tends to be faster, and the offloading of some services to CAS coupled with a deployment of Outlook in cached mode means that deploying clients at the end of slower, less reliable links can provide a very acceptable client experience.  If you can consolidate more mailboxes on fewer servers in fewer locations you should be able to demonstrate real cost saving…

Virtualization
In case you haven’t seen this already this is a great blog on You Had Me At EHLO.. - Should You Virtualize Your Exchange 2007 SP1 Environment? outlining a series of scenarios where virtualisation and specifically Hyper-V are suited to Exchange deployments. These include ‘Small Office with High Availability’,’Remote or Branch Office with High Availability’ and ‘Mobile LAN’.  A combination of data centre consolidation (as above) and Hyper-V, for those small remote offices where data centre consolidation is not possible and where it is difficult to maximise the use of the hardware, could be a very viable solution.  You should be able to meet your performance and availability requirements and still save money.

And last but not least…

Microsoft Hosted Services
Consider replacing your current cloud based hygiene, encryption and compliance solution with Microsoft Exchange Hosted Services.  OK so I would say this right because I work for Microsoft but actually this is worth a look at since if you are paying for a Microsoft Enterprise Subscription Agreement then you may not have to pay for Exchange Hosted Services at all.  Perhaps this is actually incidental since there is no requirement on having Exchange 2007 in your environment so maybe this won’t have an impact on whether your project gets approved but it at least shows willing, and the cost saving might be significant.