I doubt this service will be free once it comes out of beta. Google will definitely charge for it. They're using the free aspect to gain some traction.
I'm willing to bet there will be a level of service available for free, just like they do on Google Apps for your Domain. You can host your mail and a handful of accounts for free, but at a certain service point, it becomes 50 bucks a user.
It'll be free unless you go over the limits. The limits are high enough for most.