Is it? $15-30/head sounds like a lot, but these are employees you're paying 30k+ to in salary alone, if the addition of slack makes them 1% more efficient per month, that blows past the $15/head. Forget the tech companies with employees that easily crest 200-300k in costs after insurance and other benefits.
There's a lot of audits and regulation, in addition to tighter security, that Slack needs to prove to its enterprise customers that they can trust their employees blasting confidential information on it every day of every year.
You are comparing efficiency like in "with Slack" and "without Slack", but in fact it is a "with Slack" vs "with another messenger". On a previous job we used Telegram for work communication. It has bots and stuff, and it's blazing fast. And it's free.
There's a lot of audits and regulation, in addition to tighter security, that Slack needs to prove to its enterprise customers that they can trust their employees blasting confidential information on it every day of every year.