$1 per 10 authentications is not the route to go; better off paying per user. I have used a few Sharepoint sites that you always had to put your password in twice. If that was online, that would be two logins right there and for the life of me, I don't understand why it always asks for your password again. If you could tie desktops to use their cloud based AD I would bet every time a user unlocks their computer it would be an authentication. Use 802.1x, yep, that would be another authentication. Outlook, yep, that would be another. A user just booting and getting logged in would be about 10 authentications right there.
I bet Microsoft will bump the price to $2 and then drop them back to a $1 as no one buys into it.