How few devs do you have? Assuming a small startup of 12, you'd be able to give each dev a raise of $42 per month. Your devs would have to be severely underpaid to notice a $42/month raise.
And if you put it to a vote, "would you rather upgrade from Teams to Slack for $9 per month, or get $9 of taxable income more per month?", I think there's a very good chance you'd be switching that week.
(I don't love Slack by any means. Still, I'd pay $9/mo out of my own pocket not to use Teams.)
