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There is certainly no formula to estimate the cost, workload, time associated with a metadata / business modeling / information architecture project.
It's axiomatic to do it, as we do not question whether a country needs an army, whether we have to clean up our office desk (at least once in a while) or whether we take a shower with a certain frequency (at least I have not heard of anybody yet that has put an ROI on it). Just because their ROI cannot be quantified does not mean, it's legitimate to neglect certain tasks, sometimes common sense ("what would my mother have told me") provides a sufficient answer. Unfortunately, it has become "popular" in medium and large organizations over the past 20 years not to pay to much attention to activities without direct monetary outcome, and that is the reason why so many businesses today struggle with legal compliance, security, privacy and data quality issues.
Surely, the way out of such a situation is not to "boil" the proverbial "ocean". I suggest to collect and model metadata by business unit and/or application following the priority in which their reorganization and modernization is justified and scheduled. However, proceeding by line of business comes with the risk that existing vertical silos are maintained or newly created. It is therefore important to start with a horizontal integration modeling project that lays the foundation for the information architecture of the organization. It will show on a high level how the lines of business interact and exchange information with each other, and thus indicate where central objects are shared (Master Data Modeling / Management).