Folks -- Just sharing a few more thoughts that I'm having on Basic groups then I'll exit from this discussion.
1) Lacking most of the online features, and only storing 1 GB in attachments, the primary impact of a [new] Basic group seems to be email volume. A large group sends more emails with each post. An active, small group sends more posts, but to fewer addresses. The hard part is monitoring that in real time...not only to bill for it, but also to keep the Owner abreast of how many charges they're racking up. Until someone comes up with a good alternative, I have to go with the idea of limiting the number of subscribers.
2) If 100 isn't a good cutoff number, we need to be talking about what is. There was a post in Group_Help back in October asking for help migrating a Yahoo group of 90,000. The "right" number probably lies somewhere in between. I don't know what that might be, but even if it was 1000, we'd still be hearing from those whose groups have 1001.
Given the vacuum of information on what it costs groups.io to send 100,000 emails (or whatever), I'm in no position to argue with the current proposal. I do know that whatever the cost may be, it's coming directly out of Mark's pocket.
3) I've seen a handful of posts from people suggesting their Basic group ought to be exempt from the pricing structure because it provides a public service. This begs the question whether any Owners here have a group that provides no benefit to its subscribers. Show of hands? I didn't think so. Sorry, but such considerations cannot be part of the groups.io business model.