As Samuel mentioned, the cost is only the time and effort to integrate the tool into your process, so initial and maintance fees do not apply (should buy the book in my opinion).
As for the initial and follow on benefits, it depends on the situation of course but for the typical data mart, it provides some benefit in the form of time savings and process structure (e.g. making sure all the metadata items are populated) for the initial data mart. There would be a higher ROI benefit in follow on data marts as long as you use the tool to do the modeling and deployment (there is no easy way to reverse engineer in changes made without the tool).
There are situations where other tool choices would likely provide a higher ROI, such as projects that will have a lot of tables (e.g. 100 or more, because there is no way to create logical submodels) or projects that require reverse engineering of existing database schemas into models.