I'm not sure what you're referring to, since the audit described in the source article doesn't seem to mention any such costs.
Perhaps you're coming from a perspective where policing is presumptively bad, and we ought to avoid having police around unless we're confident there's a specific benefit in doing so? I know that many people think that way, and I'm not sure how to engage with it other than to say I don't agree. I don't think the police are perfect by any means, but I live in an area covered by ShotSpotter, and I feel safer knowing they're detecting and investigating anything that sounds like a gunshot in my neighborhood.