I always felt the way to monetize the group is through sponsorships and not costly membership fees. Use the member fee to keep out the tire-kickers and the lazy/poor consultants.
The key issue that's driving the cost question is the underlying tool(s) used to communicate (presently Socialcast). Through the growth of the group, I think we've all benefitted by bringing in new bright members. The more members the easier it has been to monetize sponsorship.
I'm not sold that the majority of the members will actively participate in much more than break-fix Q&A.
I think the best long term use of the group is as a ""lobbying"" arm to impress upon Sage the importance of fixing/improving/updating Sage 100.
I think we have a ""market top"" in the # of Sage 100 consultants who want to join the group. Maybe there's 10-20 more consultants who join but look at the marketing -- Sage isn't appearing to put any money or attention into the Sage 100 line so it's highly unlikely more consultants will be joining the fold (please, if you have evidence to support a contrary view post it here and correct me - all I'm stating is observations from looking at their website, marketing, etc).
I don't recall advocating for any type of special fee for founding/original/long-time members.
Just opening a section for a new product isn't going to produce activity. I think you'll find a need to seed it with 5 to 15 regularly active members who post current topics about the product. Without that type of activity, you will see members log in and log out (See Sage 300). I am very concerned that the channel for most of the products you mention is going to become competitive which in turn could cause companies to pull back on their collaboration - but I could be wrong.
Comparing to ITA and SLA is not apples to apples as those groups are organization/development/executive level. A better comparison is probably to the Microsoft user group fee structure (consultants pay a lot, customers very little). Before you grab a torch and pitchfork and hit reply with the flame war about how 90 Minds will never let in customers ----- consultants = pay lower dues as they are the customer in this model, --- vendors = pay higher dues as they are the consultant in this model.
In my view what you have here is a great break-fix resource for consultants still active in Sage 100. Will it expand to Acumatica, Intacct and anything else that Sage might buy? Perhaps, however, think of it more of a ""build it and they will come"". Before you let in new members to another product section make sure it's active or the new members will leave as quickly as they came.
Nearly everyone in the other products mentioned is still in startup mode and, in my view, very competitive. I question how much they will collaborate.
That being said, I'd concentrate on how a group of 90 Minds size can monetize through sponsorships. To that extent, I think @DarcyBoerio has a lot more experience than any of us. I'd look to use the least costly back-end and give strong consideration to how much the typical consultant uses any of the tools beyond the ability to post and get a second opinion on a tech issue. There might be a big opportunity to get further into bed with Sage but only at the executive level (don't bother talking to the low leve). And lastly, keep an eye on what Sage may unveil for their new partner portal. They have talked about creating a new discussion area though we all know how the last portal compared (actual vs promised).
Good luck!