Wow #yammer shot me a quote of ""about $1,000 to $1,500"" for 100 users though that was only for one external network and I don't think it was for the full Yammer. He wasn't totally clear.
Maybe they have a way to activate different feature piecemeal.
Looks as if the true discounted rate starts at about $15 per year. My guess is that for large groups that these guys go $5 to $10 per year per user.
Just an FYI - I still think SC is a better overall deal.