Old thread, but I ended up here while doing new research, so might as well respond in case others do too.
The above is not right. You can add aliases to existing accounts for free. Essentially, it's $50 per "login" account per year, and that can have lots of aliases. You can also have groups (free again), that allow you to have many people using the same address - handy for things like "sales@"
As for the subject of the thread, that's what I've been researching. What you can't do via an api yet is sign up new google apps domains. There's always talk of this, but it doesn't seem to exist for the moment.
