It'd be like joining a stock trading slack channel where you'd get access to top information that no one else would know. How much would you be willing to pay?
It's quite clever.. but no different than subscribing to some "Insider Journal" or something like that.
I think SlackPass even helps with that. https://slackpass.io/
I've known communities that are paywalled. Instead of thinking it as a "slack channel that costs $$", the value prop is "it's a closed community whose entry is $$". There have been vBulletin forums where if you were a paid subscribers, they got you access to subscriber-only subforums.
Couple of examples currently:
1. Product Management HQ community: $25 to join. Slack channel is there as well. https://www.productmanagerhq.com/join-the-community/
2. Product People Club: [it was paid. don't know current price. entry seems closed right now]. https://productpeople.club/