Why do you think that Google's costs are less when sending messages inside their "intranet" versus sending messages to and from the outside world? Their network is huge and it's unlikely that your users XMPP accounts are managed by a server in the same data centers as your App Engine instances. So it's not costing Google more, it probably costs them the same.
Also keep in mind that if your time is not free, App Engine may be cheaper than EC2. At some point, renting out a colo and buying your own hardware is cheaper than EC2. And then at another point, it's cheaper to build and manage your own data center. It's all a continuum of effort and upfront costs vs. monthly costs.
Also keep in mind that if your time is not free, App Engine may be cheaper than EC2. At some point, renting out a colo and buying your own hardware is cheaper than EC2. And then at another point, it's cheaper to build and manage your own data center. It's all a continuum of effort and upfront costs vs. monthly costs.