I have been on this topic for ages and I truly believe that this is an answer that could only be answered once I have experienced it. Here we go. I think the total cost of ownership is something that is very difficult to calculate. First of all, alot of the “gardeners” or “champions” I have seen and worked with work extra long hours to ensure that the platform is “nice” and “clean” and still perform their day jobs extremely well. So if effort is invested after hours could we consider this as part of the TCO equation? Some might argue there is a opportunity cost involved and they could be working on real work after hours. But if they are not motivated by their day job scope and prefers tending the “garden” then would they work after hours? I do not think there is an answer to this.
What I have seen is that champions move in and take control of the maintenance of the platform and look after it. All of them have day jobs and generally their day jobs is not affected by their extra work load. IT seldom gets invloved in maintenance other than the standard harddrive replacement, server upgrades and stuff like that which applies to all software that is running in house. These costs can be calculated easily. It seems to me that the human maintenance costs is relatively low as well - as long as you have the right people maintaining it. Do you need a dedicated team to maintain a large Enterprise deployment? Maybe.
I think the TCO arguement for Enterprise 2.0 is still very new and unclear. I do not know all the answers but the above is something I have observed.
More...