For anyone who disagrees with this post: please leave a comment or send me an email. If I am incorrect about something, I will modify the post so that I am not spreading misinformation. And I would love to have a dialog on the topic.
There are many articles that advise doc managers about how to calculate return on investment for potential DITA adoption. Most of these articles seem to be written by consultants who make money by helping companies set up DITA systems: they have a vested interest in making DITA look beneficial. Also, they tend to help out during the initial migration and might not be around when some of the costs kick in: they simply might not be aware. Finally, they might deal mostly with large companies where large expenditures do not seem excessive. For whatever reasons, the literature seems to be underestimating the true cost of moving to DITA.
There can be a lot of costs related to DITA adoption. Here are some that might affect you. (Different implementations will vary somewhat.)
Most of us know about the cost of a CMS, which can set you back over $250,000 (or might be a lot less). You can do without the CMS, but DITA is designed for use with a CMS and you need one to get the full benefits. But the CMS is just the start.
In the early stages of your migration to DITA, you will likely need to hire DITA specialists (the consultants I mention above) to help you plan and set up your system.
You'll likely need more inhouse tools developers, and you may need developers with different skills than you currently have. This is not just to set up the new publishing system and so on, but also to troubleshoot publishing problems, adapt to new browser versions, and address all the bugs and glitches. In my experience there are all sorts of problems that crop up with the relational database (CMS), and also with the ditamaps, formatting of outputs, and many other things. Part of the problem is that the DITA Open Toolkit is notoriously difficult (and could require extra expense for things like a rendering engine). Some of the tools designed to work with DITA are arguably not quite up to speed yet. Your tools developers will also spend a lot more time helping writers.
(If you don't move to a CMS, but use something like FrameMaker and WebWorks ePublisher, you may find that you have a lot more headaches in producing docs without much in the way of DITA benefits.)
You need extremely skilled information architects to create a reuse strategy, engage in typing exercises, and design and edit your ditamaps. This isn't a skill set that people currently in your organization can easily acquire. Even most information architects have trouble adequately mapping topics. For a discussion of the sorts of challenges they'll face, see this series of posts; the moral is: if you don't have skilled architects working on your system, you may end up with Frankenbooks that are not particularly useful for your readers. I raise some additional topic reuse problems in an earlier post: link.
You'll need to spend significant time developing new processes, policies, and internal instruction manuals.
Your team will have to undergo intensive training. In coming years, new writers you hire will also need training. I have found that writers can move to Docbook XML with very little training, but DITA requires a great deal of training, not just for the CMS, but also to learn how to use ditamaps, reltables, and so on.
The migration of content will likely be quite time consuming, with manual work required to correct tagging that doesn't convert automatically, mapping, and a complete indepth edit.
Your writers will need to spend more time on non-writing activities. This can greatly reduce their productivity. Working with a relational database, especially an opaque one like a CMS, is much more time consuming than checking files out of a version control system. Creating reltables is a lot more work than adding links. Coordinating topics is a lot more work than designing and writing a standalone deliverable. Plus, there is a lot more bureaucracy associated with DITA workflows.
With most DITA implementations, topics exist in a workflow that only starts with the writer. You'll probably need more editors and more software.
You'll also probably need more supervisors. The DITA literature emphasizes the importance of assimilating writers to the new regime and then monitoring their attitudes. Pre-DITA, writers were project managers for their own content; with DITA they have to learn to hand that responsibility off to others.
There are some organizations, such as ones that have to cope with hundreds of versions of a hardware product, that have a clear ROI for DITA. But many (most?) organizations could find that DITA doesn't so much save money as redistribute money. Where before you spent the lion's share of your doc budget on salaries for writers, now writer salaries will be a much smaller proportion of your budget. In many cases, companies could find themselves facing higher costs than pre-adoption: they will never see return on their investment. And given the complexities of using DITA, ongoing hassles and escalating costs, some companies are going to find themselves having to ditch DITA and go through an expensive migration to another system.