I think you are seeing it from the strongly-VC-backed startup perspective.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.