Here's the formula we use when a team tells us Slack (or Teams) feels heavier than it should: team size × interrupting messages per person per day × $12 × 20 working days. A 5-person team at 10 interruptions a day each lands at $12,000 a month.
The $12 is what one interruption costs: 11 minutes of attention and re-focus, priced at a $65/hr blended rate. Ten of those a day is close to two hours of the working day — which is why the total sounds wrong until you sit through a Tuesday. The point isn't to quit chat tools. It's to stop spending them on things a workflow should carry: status requests, file chases, handoff questions. Every one of those has an automated answer that also produces a written record.