Spoke to the Yammer sales guy today. Sounds like he would give us non-profit pricing if we were to move that way. My guess is about 20% off ($60 per year per person is the full rate) but we'd probably need to confirm.
He would bill annually so we could collect one amount each year. Any trial people who came in during the year can be added for the part year.
One thing we might want to consider is to build in 3 accounts per 90 Minds member so that incents individual members to add in additional people from their company. I think we have an untapped market of existing member employees who probably could benefit greatly from participating here -- whether it's support, marketing, administrative.
With a Yammer paid account he said we would not all need the same domain account to login.
Basically I see Yammer as a better solution primarily for the ability to add in external networks, better mobile clients (support for multiple Yammer accounts for those of us with company accounts as well).
I think we should not consider Yammer/Socialcast type systems as our primary KB but instead should add to an external KB with data pulled from our communications system.