Hiring ISTQB certified testers pays back mainly in two places: fewer defects reach customers, and the team stops losing hours to arguments about words. A defect caught while a requirement is still being read costs a conversation. The same defect found in production costs triage, a hotfix, regression testing, support calls and sometimes a customer. That gap is the business case, and a certificate is only the first, cheap signal that someone has been trained to close it.

Where does a certified tester save an employer money before release?
Put the same defect at three points in a project and follow the bill. Found while the requirement is reviewed, it costs a few minutes of a business analyst's time and a corrected sentence. Found during system testing, it costs a developer to reproduce, fix and re-check it, plus a tester to retest and rerun the tests around it. Found in production, it adds everything the first two have, and then an incident, a deployment outside the normal cycle, a message to customers and the time of people who were meant to be building something else. No published figure is needed to see the shape of the curve, and none is invented here: every stage adds people, and each one adds more than the stage before.