Like your cable bill, the internal cost of your servers tends to get bloated over time. When was the last time you checked your bill? I was working with one customer whose charges for servers was over $600 K. Upon looking at the bill, they were paying for servers that were not Domino servers, and paying for servers that didn't exist anymore.
That is a start, but what is the next step? A good way to consolidate servers is too look at the databases that are being used. And the key thing is to look at databases not being used by people. The unfortunate part of the activity reported by the Catalog is it lumps all activity together. So, you can have a mail file that shows 'activity', but it is simply the router dumping spam in that mail file. Likewise, you can have an agent running in a database that users don't access. Thus, activity, but no users. The good news is that you can use the Activity Logging feature of Domino to generate useful information. The information provided by Domino is not real useful by itself, but I have a tool that can consolidate the information and give you the information you need to identify databases not being used by people. Let me know if you want more information.
With this list in hand, you can start to identify servers that can be eliminated by consolidating the databases on fewer servers. And when you start to consolidate servers, you should think about a tool like Panagenda's Marvel Client that can update all your users desktops to point to the changed server for the databases.