This is a horrible way to think about it, slack doesn't need to deliver 4x the value, it just needs to deliver $6 worth of extra value per month.
Even at the very cheapest end of the spectrum, a developer is going to cost at least $60 an hour, fully loaded. If slack can make them 6 minutes more productive a month (ie: 12 seconds a day), then slack is worth it. These are productivity differences that aren't even measurable in practice which means the rational thing to do is to just be price insensitive and adopt the tool that works best, regardless of cost.
Even at the very cheapest end of the spectrum, a developer is going to cost at least $60 an hour, fully loaded. If slack can make them 6 minutes more productive a month (ie: 12 seconds a day), then slack is worth it. These are productivity differences that aren't even measurable in practice which means the rational thing to do is to just be price insensitive and adopt the tool that works best, regardless of cost.