The cost shows up in places nobody labels as data: reworked invoices, undeliverable shipments, analysts reconciling two reports by hand, a marketing spend against duplicated contacts, a regulatory resubmission. Each is small and each has an owner who has stopped noticing it, which is why the total is invisible until someone adds it up. Adding it up is usually the most persuasive thing a governance program ever does.
In practice. Cost three specific failures rather than estimating a global figure. Hours × loaded rate × frequency, plus any direct spend. Three credible, defensible numbers from processes the business recognizes will do more for a business case than a benchmark percentage of revenue.
Where it goes wrong. The figure is sourced from a vendor study — "bad data costs companies 15–25% of revenue" — and presented as this company's number. A CFO will discount it in one sentence, and the credibility does not come back. Your own three examples, however modest, survive the question "where did that come from".