I actually looked at that. But for what I read, the cost is then based on user base, 8$ per user per month at a max of 1000$ per month. So we would probably reach that on the first month...
You may want the extra services that this gives, but it is not the standard App Engine offering.
If you are not using App Engine for Business, the costs are for CPU and bandwidth, etc., and not for users, and so should be significantly less than that.
edit: The standard pricing structure is at http://code.google.com/appengine/docs/billing.html and there is a free quota as well which is pretty generous.