Investment in an ITSM project, or which more later, is often a mix of funds already allocated and new funds. Take internal staff costs for example. You are paying people's salary whether they are doing their day job or working on an ITIL project. Only very rarely have I come across an IT department that employs additional contractors to backfill for people engaged on the ITIL project.
That, of course, tells us somethign about how well utilsed some of those people are if they can be seconded to a longterm project with no need for resources to replace them.
So what are the costs of doing ITSM?
Project Management overhead
The cost of the new processes put in place
Reduce any of these without impacting the benefit and we've improved ROI