There's a lot of ground to cover in this topic, but having read through it now I do have some 1st impression comments.
I agree with what many have said about 100 being too low an absolute limit for a Basic group, and with those who've suggested a per-member price above 100 as a useful solution, for you and for many (not all) groups.
That comes from a feeling that the feature-set differences between Basic and Premium are likely orthogonal to the number of members in the group, as far as the group's purpose, culture, and needs are concerned.
The pricing model you've proposed suggests that the feature set weighs more heavily than the transmission costs in your total cost outlook ($2.20/member for the Enterprise feature set versus $0.55/member for the Premium feature set). I'm skeptical of that inference, but going with it anyway I'd suggest a yet lower rate per member over 100 for the Basic feature set.
Something I just noticed, that I don't see mentioned elsewhere, is that your original post in this topic doesn't actually mention "Basic" groups, instead it says "Free". Is this a mere change in terminology, or are there other implications (besides the 100 member limit)?
In particular, what does "Single mailing list only" mean in this context? Is it another way of saying "No Subgroups", or does it mean something broader?