http://www.npr.org/sections/money/2015/09/23/442921757/episo...
http://www.npr.org/sections/money/2015/09/23/442921757/episo...
I feel I can pretty authoritatively answer "Betcha not" here. </cofounder>
It's trivial to justify Slack's pricetag at any software company. Your entire team lives in it continuously and it becomes the nerve center of the company... for freaking pennies. It's ludicrous. If you add it up the non-Starfighter companies I run spend like $50k a year on SaaS/hosting/etc ("everything with an API attached") of which Slack is like ~1%, and probably the most underpriced-relative-to-value service which does not quote its pricing in picodollars.
IRC, mattermost, email, Gitlab, Wekan, Skype/Google Hangouts/BigBlueButton.... there are plenty of tools to communicate without spending loads of cash.
Seriously, Slack was the EASIEST things to justify as an expense when we proposed it to upper management earlier this year. And they are known to be very stingy.
I might hypothetically use an open source tool instead of Slack because it's not worth $2000 of productivity for 15 people per year.