I would be ready to pay $3/month for screenhero standalone or a pay-per-use. I'm not interested in Slack, the use cases are clearly different.
I think Slack missed or rather killed an opportunity here, they could have kept Screenhero (and use it to upsell Slack) and also integrated it in Slack. Not only that but it took them over 2 years to make this integration and in the meantime Screenhero remained invite-only.