The problem is that there are many useful tools and services that most of us are using for free: Text editors, browsers, email, forum software, all sorts of apps, cloud storage etc. It's not realistic to pay 5$ or 12$ a month for each.
Not saying that this is Slack's problem, and we really should cultivate more community organized tooling based on Open Source. But I would say 12$/month is a lot for OP's usecase.